See #863

This commit strives to make the usage for new PyPDF2 users easier by following
PEP8 naming schemes. It's mostly about camelCase method names being converted to
snake_case. Other changes make the public interface of PyPDF2 smaller and thus
easier to discover.

This commit does not introduce any breaking changes as the old modules /
classes / method signatures are still present. They have now deprecation
warnings and the docs show that those are considered deprecated.

If a property and a getter-method are both present, use the property.

Module level changes
--------------------

- utils ➔ _utils: The module is renamed to '_utils' to indicate that it should
                not be used by PyPDF2 users. It's only meant for PyPDF2 itself.
- The 'pdf' module was removed. Most classes / functions are now either in
  '_utils' or in 'generic'.


Core classes
------------

- PdfFileReader➔ PdfReader (strict=False is new default)
- PdfFileWriter➔ PdfWriter
- PdfFileMerger➔ PdfMerger (strict=False is new default)

PdfReader
---------

- writer.getPage(pageNumber) ➔ writer.pages[page_number]
- writer.getNumPages() ➔ len(writer.pages)
- getPageLayout / pageLayout ➔ page_layout
- getPageMode / pageMode ➔ page_mode
- getIsEncrypted / isEncrypted ➔ is_encrypted
- getDocumentInfo ➔ metadata

PdfWriter
---------

- writer.getPage(pageNumber) ➔ writer.pages[page_number]
- writer.getNumPages() ➔ len(writer.pages)
- getPageLayout / setPageLayout / pageLayout ➔ page_layout
- getPageMode / setPageMode / pageMode ➔ page_mode

Page
----

- mediabox / trimbox / cropbox / bleedbox / artbox:
    - getWidth, getHeight  ➔ width / height
    - getLowerLeft_x / getUpperLeft_x ➔ left
    - getUpperRight_x / getLowerRight_x ➔ right
    - getLowerLeft_y / getLowerRight_y ➔ bottom
    - getUpperRight_y / getUpperLeft_y ➔ top
    - getLowerLeft / setLowerLeft ➔ lower_left property
    - upperRight ➔ upper_right
- Add Transformation class to make it easy to create transformation matrices
- add_transformation and merge_page should be used instead of:
    - mergeTransformedPage
    - mergeScaledPage
    - mergeRotatedPage
    - mergeTranslatedPage
    - mergeRotatedTranslatedPage
    - mergeRotatedScaledPage
    - mergeScaledTranslatedPage
    - mergeRotatedScaledTranslatedPage

See the CHANGELOG for a full list of changes

This release adds a lot of deprecation warnings in preparation of the
PyPDF2 2.0.0 release. The changes are mostly using snake_case function-, method-,
and variable-names as well as using properties instead of getter-methods.

Maintenance (MAINT):
-  Remove IronPython Fallback for zlib (#868)

Full Changelog: 1.27.12...1.27.13

* Make the `PyPDF2.utils` module private
* Rename of core classes:
  * PdfFileReader ➔ PdfReader
  * PdfFileWriter ➔ PdfWriter
  * PdfFileMerger ➔ PdfMerger
* Use PEP8 conventions for function names and parameters
* If a property and a getter-method are both present, use the property

In many places:
  - getObject ➔ get_object
  - writeToStream ➔ write_to_stream
  - readFromStream ➔ read_from_stream

PyPDF2.generic
  - readObject ➔ read_object
  - convertToInt ➔ convert_to_int
  - DocumentInformation.getText ➔ DocumentInformation._get_text :
    This method should typically not be used; please let me know if you need it.

PdfReader class:
  - `reader.getPage(pageNumber)` ➔ `reader.pages[page_number]`
  - `reader.getNumPages()` / `reader.numPages` ➔ `len(reader.pages)`
  - getDocumentInfo ➔ metadata
  - flattenedPages attribute ➔ flattened_pages
  - resolvedObjects attribute ➔ resolved_objects
  - xrefIndex attribute ➔ xref_index
  - getNamedDestinations / namedDestinations attribute ➔ named_destinations
  - getPageLayout / pageLayout ➔ page_layout attribute
  - getPageMode / pageMode ➔ page_mode attribute
  - getIsEncrypted / isEncrypted ➔ is_encrypted attribute
  - getOutlines ➔ get_outlines
  - readObjectHeader ➔ read_object_header (TODO: read vs get?)
  - cacheGetIndirectObject ➔ cache_get_indirect_object (TODO: public vs private?)
  - cacheIndirectObject ➔ cache_indirect_object (TODO: public vs private?)
  - getDestinationPageNumber ➔ get_destination_page_number
  - readNextEndLine ➔ read_next_end_line
  - _zeroXref ➔ _zero_xref
  - _authenticateUserPassword ➔ _authenticate_user_password
  - _pageId2Num attribute ➔ _page_id2num
  - _buildDestination ➔ _build_destination
  - _buildOutline ➔ _build_outline
  - _getPageNumberByIndirect(indirectRef) ➔ _get_page_number_by_indirect(indirect_ref)
  - _getObjectFromStream ➔ _get_object_from_stream
  - _decryptObject ➔ _decrypt_object
  - _flatten(..., indirectRef) ➔ _flatten(..., indirect_ref)
  - _buildField ➔ _build_field
  - _checkKids ➔ _check_kids
  - _writeField ➔ _write_field
  - _write_field(..., fieldAttributes) ➔ _write_field(..., field_attributes)
  - _read_xref_subsections(..., getEntry, ...) ➔ _read_xref_subsections(..., get_entry, ...)

PdfWriter class:
  - `writer.getPage(pageNumber)` ➔ `writer.pages[page_number]`
  - `writer.getNumPages()` ➔ `len(writer.pages)`
  - addMetadata ➔ add_metadata
  - addPage ➔ add_page
  - addBlankPage ➔ add_blank_page
  - addAttachment(fname, fdata) ➔ add_attachment(filename, data)
  - insertPage ➔ insert_page
  - insertBlankPage ➔ insert_blank_page
  - appendPagesFromReader ➔ append_pages_from_reader
  - updatePageFormFieldValues ➔ update_page_form_field_values
  - cloneReaderDocumentRoot ➔ clone_reader_document_root
  - cloneDocumentFromReader ➔ clone_document_from_reader
  - getReference ➔ get_reference
  - getOutlineRoot ➔ get_outline_root
  - getNamedDestRoot ➔ get_named_dest_root
  - addBookmarkDestination ➔ add_bookmark_destination
  - addBookmarkDict ➔ add_bookmark_dict
  - addBookmark ➔ add_bookmark
  - addNamedDestinationObject ➔ add_named_destination_object
  - addNamedDestination ➔ add_named_destination
  - removeLinks ➔ remove_links
  - removeImages(ignoreByteStringObject) ➔ remove_images(ignore_byte_string_object)
  - removeText(ignoreByteStringObject) ➔ remove_text(ignore_byte_string_object)
  - addURI ➔ add_uri
  - addLink ➔ add_link
  - getPage(pageNumber) ➔ get_page(page_number)
  - getPageLayout / setPageLayout / pageLayout ➔ page_layout attribute
  - getPageMode / setPageMode / pageMode ➔ page_mode attribute
  - _addObject ➔ _add_object
  - _addPage ➔ _add_page
  - _sweepIndirectReferences ➔ _sweep_indirect_references

PdfMerger class
  - `__init__` parameter: strict=True ➔ strict=False (the PdfFileMerger still has the old default)
  - addMetadata ➔ add_metadata
  - addNamedDestination ➔ add_named_destination
  - setPageLayout ➔ set_page_layout
  - setPageMode ➔ set_page_mode

Page class:
  - artBox / bleedBox/ cropBox/ mediaBox / trimBox ➔ artbox / bleedbox/ cropbox/ mediabox / trimbox
    - getWidth, getHeight  ➔ width / height
    - getLowerLeft_x / getUpperLeft_x ➔ left
    - getUpperRight_x / getLowerRight_x ➔ right
    - getLowerLeft_y / getLowerRight_y ➔ bottom
    - getUpperRight_y / getUpperLeft_y ➔ top
    - getLowerLeft / setLowerLeft ➔ lower_left property
    - upperRight ➔ upper_right
  - mergePage ➔ merge_page
  - rotateClockwise / rotateCounterClockwise ➔ rotate_clockwise
  - _mergeResources ➔ _merge_resources
  - _contentStreamRename ➔ _content_stream_rename
  - _pushPopGS ➔ _push_pop_gs
  - _addTransformationMatrix ➔ _add_transformation_matrix
  - _mergePage ➔ _merge_page

XmpInformation class:
  - getElement(..., aboutUri, ...) ➔ get_element(..., about_uri, ...)
  - getNodesInNamespace(..., aboutUri, ...) ➔ get_nodes_in_namespace(..., aboutUri, ...)
  - _getText ➔ _get_text

utils.py:
  - matrixMultiply ➔ matrix_multiply
  - RC4_encrypt is moved to the security module

* STY: Adjust code/docs in several places to make it more similar to the 2.0.0 branch
* MAINT: Remove excessive <py36 warnings

Bug Fixes (BUG):
-  Incorrectly show deprecation warnings on internal usage (#887)

Maintenance (MAINT):
-  Add stacklevel=2 to deprecation warnings (#889)
-  Remove duplicate warnings imports (#888)

Full Changelog: 1.28.0...1.28.1