sphinx.domains.python.PyXrefMixin.make_xrefs should be consistent with _parse_annotation

Sub-issue split off from #9523.

The tensorstore documentation theme changes sphinx.domains.python.PyXrefMixin.make_xrefs to just use _parse_annotation, so that the formatting of types is the same in both signatures and in the field lists (e.g. "parameters", "return type", "raises" listings):

https://github.com/google/tensorstore/blob/1a59fcb310bc1feb13569f03f7134b4c3a5fa5f4/docs/tensorstore_sphinx_ext/autodoc.py#L76

Currently make_xrefs has a separate implementation, perhaps to accommodate more free-form syntax used in older parameter type specifications before the introduction of type annotations, like "int or float":

https://github.com/sphinx-doc/sphinx/blob/ba2439a105e5a68a44dd3f839e86332889cce451/sphinx/domains/python.py#L331

I think the older-style "int or float" style should be discourage now that type annotations are so widely used. However, if it is desired to still support them, make_xrefs should still dispatch to _parse_annotations so that real type annotations are displayed consistently.

Start in sphinx/domains/python.py at PyXrefMixin.make_xrefs and _parse_annotation, then compare the tensorstore autodoc.py reference linked in the issue. Determine whether older free-form specifications such as "int or float" remain supported while making real annotations render consistently. Done means the two paths share consistent formatting, with relevant existing behavior preserved.