autodoc: Add support for typing.TypeAlias declarations

[dalcinl]

Purpose

The new implementation of autodoc expects the target of autotype to be a proper type statement as introduced in
Python 3.12. Legacy type aliases are deprecated since 3.12 in favor of type statements. However, they are still supported and according to Python docs, there are no current plans for removal.

Without the first commit in this PR, Python code like the following

from typing import TypeAlias
IntOrStr: TypeAlias = int | str

cannot be documented with the autotype directive:

.. autotype:: IntOrStr

as this will lead to a runtime failure.

Traceback
=========

      File "<rootdir>/sphinx/sphinx/ext/autodoc/_dynamic/_loader.py", line 469, in _make_props_from_imported_object
        obj.__value__, render_mode, short_literals=short_literals
        ^^^^^^^^^^^^^
    AttributeError: 'typing.Union' object has no attribute '__value__'. Did you mean: '__le__'?

[AA-Turner]

@dalcinl please can you split this into two PRs?

A

[dalcinl]

@AA-Turner Done. This PR is now for the autodoc fix exclusively, which is IMHO the most pressing one. The fix for autosummary is #14174.

[AA-Turner]

I think this is a new feature, rather than a bug fix. Recategorised accordingly.

A

[dalcinl]

Can we still get this change in the next patch release 9.0.5?

[AA-Turner]

The next version of Sphinx will be 9.1, we've closed patch releases for the 9.0 series.

A