Use format_annotation to render class attribute type annotations

[hoodmane]

Resolves #298.

This is a bit hard to do because autodoc doesn't emit any events to help us adjust the type rendering for attributes and because the type annotation is currently converted to text and then rendered from text by the PyAttribute directive.

The system in autodoc for attribute rendering is:

1. In AttributeDocumenter.get_directive_header, call stringify_annotation to render the annotation to a string
2. add :type: stringified annotation to the directive header of PyAttribute
3. When sphinx.domains.python.PyAttribute sees the :type: it calls format_annotation which attempts to convert the string representation of the type into a more fancy representation with cross referencing

Notes:
2. we don't get an opportunity to adjust the directive header with autodoc-process-docstring or some other event
3. there isn't enough information retained in the string for _inject_types_to_docstring to work.

So my approach is:

1. Patch AttributeDocumenter.get_directive_header to use format_annotation instead of stringify_annotation. Add a prefix to the output to indicate that it is rst not a plain text type annotation.
2. Patch PyAttribute so that when it sees the prefix, it formats the output as rst rather than trying to handle it as a plain text type annotation.

As future proofing, we check that the patch targets exist and bail if they don't exist. If they move this patch should gracefully decay into a no-op rather than crashing.

[gaborbernat]

You could use none just need to annotation it str | None, no?

[hoodmane]

That makes a type error when I try to use it unless I add an extra assert. Empty string is falsey and this makes the type checker happy without adding the assert.