The restify() helper crashes when ForwardRef is passed.

Fix #8559: AttributeError is raised when using ForwardRef

Fixes #8568. Ignore TypeError from getslots in isslotsattribute

…nt class

The instance attributes on subclasses are shown on the document of
parent class unexpectedly because of autodoc modifies `__annotations__`
in place.  This fix creates a copy of `__annotations__` attribute and
attach it to the subclass.

Fix #8567: autodoc: Instance attributes are incorrectly added to Parent class

Linkcheck organizes the URLs to checks in a PriorityQueue. The items are
tuples (priority, url, docname, lineno).

Tuples where the lineno is `None` are not comparable with tuples that
have an integer lineno, and PriorityQueue items must be comparable (see
https://bugs.python.org/issue31145).

Fixes an issue when a document contains two links to the same URL, one
with an int line number and the other without line number metadata (such
as an image :target: attribute).

Using 0 instead of None to represent no line number should not lead to
observable changes, the result logger only logs the line number when it
is truthy.

Close #8565

Ensure linkcheck items are comparable

On the refactoring of Documenter.add_content() method, the
autodoc-process-docstring event is emitted to the alias classes (called
as doc_as_attr in our code) unexpectedly.  Before the change, it has
never been emitted.

The event causes that extensions (ex. numpydoc) confused.  So this
reverts the change temporarily (during 3.4.x).

refs: #8533

…for_alias_classes

Fix #8566: autodoc-process-docstring is emitted to the alias classes

It should be compared by `is` keyword instead.

Fix #8583: autodoc: Unnecessary object comparision via ``__eq__`` method