Initial support for PEP 695 type aliases

[mmatous]

Purpose

Hi, this PR includes support for documenting PEP 695 type aliases. The main problem was that while looking for docstrings, Sphinx's parser didn't recognize ast.TypeAlias as visitable (for doc comment, missing visit_TypeAlias()) and TypeAliasType object wasn't recognized as something documentable when encountering docstring in visit_Expr() (for docstrings).

The rest of it is mostly just me guessing how stuff should be rendered into ReST and trying to ensure Python 3.11 compatibility.

I put the relevant autodoc parts into ClassDocumenter since that's where code for other type alias variants lives.

Couple of caveats:

• Type aliases in signatures don't get cross-linked in HTML. I could use some pointers here. I think the ReST is mostly fine and the problem lies in HTML gen.? What should I do about that?

• Generic type aliases do not get rendered as such. E.g. type A[T] = list[T] yields only type A = list[T] in resulting HTML. This seems expected, since there is no supported syntax for type params in py:type, unlike py:method

• Does not include support for PEP 695 type parameters in generic classes (class ClassA[T: str]:) or functions/methods (def func[T](a: T, b: T) -> T:). I thought about it briefly and it seems more complicated than I'm willing to tackle rn.

• How should I go about ruff accepting the test file? Add exemption to pyproject? SyntaxError bc of targeting Py3.11 can't be suppressed.

References

• requested in autodoc: Add support for PEP 695's explicit type aliases #11561