Render TypeVars as text in restify

[gaborbernat]

Fixes #14502.

restify() emits a :py:obj: cross-reference for a TypeVar, whereas stringify_annotation() renders TypeVars as plain text. A TypeVar is not a documentable target, so that reference is never resolvable. With show-inheritance, documenting a class that derives from Generic[_T] (where _T is an undocumented, e.g. private, type variable) therefore fails a nitpicky / -W build:

py:obj reference target not found: mod._T [ref.obj]

This renders TypeVars by name in restify() too, making it consistent with stringify_annotation() and removing the dangling reference. The existing test_restify_type_hints_typevars assertions are updated to the text rendering, and a regression note is added to CHANGES.rst.

Minimal reproducer (passes after this change):

# mod.py
from typing import Generic, TypeVar
_T = TypeVar("_T")
class Foo(Generic[_T]):
    """A generic class with a private TypeVar."""

# conf.py
extensions = ["sphinx.ext.autodoc"]
nitpicky = True
autodoc_default_options = {"members": True, "show-inheritance": True}

.. automodule:: mod
   :members: