Move from top level to more appropriate generic folder. Also rename,
to remove the redundant "helpers".

Add first-class support for annotation replies (PDF 1.5) by extending
MarkupAnnotation with three new parameters:

- in_reply_to: reference to the parent annotation, sets /IRT
- reply_type: "R" (reply, default) or "Group", sets /RT
- annotation_name: sets /NM for annotation threading (auto-generated
  UUID when in_reply_to is set)

Closes #2065.

Closes #3668.

Set faulthandler timeout for PyPy tests. Hopefully avoids long runtimes and provides more context where the tests get stuck there.

Relates to #3667.

It currently is unclear how to correctly handle the links in this case,
as `zip()` in strict mode would raise due to the current mismatches
which seem to occur for legitimate files. In most of the cases, there
will be no new annotations and thus always an empty list as the return
value.

It seems like the `loadfile` strategy to not use multiple workers for a test file seems to be more reliable than the tests on the current *main* branch. Let's hope that this finally brings us closer to CI not being unreliable with PyPy any more.

## What's new

### Security (SEC)
- Limit allowed `/Length` value of stream  (#3675) by @stefan6419846

### New Features (ENH)
- Add /IRT (in-reply-to) support for markup annotations (#3631) by @costajohnt

### Documentation (DOC)
- Avoid using `PageObject.replace_contents` on PdfReader (#3669) by @stefan6419846
- Document how to disable jbig2dec calls by @stefan6419846

[Full Changelog](6.7.5...6.8.0)