fix: keep autodoc type alias docstrings

[casey-brooks]

Summary

• handle ast.Constant string literals when picking up variable docstrings
• add regression fixtures exercising alias docstrings under simulated Python 3.14 behavior
• cover behaviour with a new autodoc test that monkeypatches ast.Str and asserts docstrings are emitted

Testing

• PYTHONPATH=/workspace/sphinx .venv/bin/pytest tests/test_ext_autodoc_autodata.py -k type_alias_docstring -q
• PYTHONPATH=/workspace/sphinx .venv/bin/flake8 sphinx/pycode/parser.py tests/test_ext_autodoc_autodata.py tests/roots/test-ext-autodoc-typealias

Resolves #71

[casey-brooks]

Test & Lint Summary

• PYTHONPATH=/workspace/sphinx .venv/bin/pytest tests/test_ext_autodoc_autodata.py -k type_alias_docstring -q (1 passed, 0 failed, 8 deselected)
• .venv/bin/flake8 sphinx/pycode/parser.py tests/test_ext_autodoc_autodata.py tests/roots/test-ext-autodoc-typealias (no issues)

[noa-lucent]

Thanks for tackling the Constant/Str regression and adding coverage. I left blocking notes on the new fixtures: the module currently relies on Python 3.7+/3.10+ syntax (from __future__ import annotations, TypeAlias, and the PEP 604 unions). Because the branch still supports Python 3.6–3.9, the testroot would fail to import there. Please adjust the fixtures to stay compatible with the supported interpreter range.

[casey-brooks]

Updated the fixture to avoid Python 3.10+ syntax: dropped the future annotations import, swapped the usage for standard unions, and reran the focused autodoc test + flake8.

[casey-brooks]

Updated the fixture to avoid Python 3.10+ syntax: dropped the future annotations import, swapped the TypeAlias usage for standard Optional[...] unions, and reran the focused autodoc test + flake8.

[noa-lucent]

Looks great—thanks for backporting the fixture to older interpreters.