This adds a configuration option that allows the user to rewrite module names if
needed. Resolves tox-dev#473. Note that we also rewrite the names of `_io` and
`typing_extensions` internally. One benefit of this hook is that if other
similar rewrites are needed, users will be able to immediately add them without
having to patch sphinx_autodoc_typehints itself.

One disadvantage is that by default, having a function in the config prevents
caching. I think this can be handled with the following slightly inelegant hack:

```py
class ModuleNameRewriteHook:
   version: int

   def __eq__(self, other):
      return type(self) == type(other) and self.version == other.version

   def __init__(self):
      self.version = 2

   def __call__(self, module):
      # logic here
      # Make sure to bump version if you edit this so that sphinx will rerun.
      return module

typehints_fixup_module_name = ModuleNameRewriteHook
```

See sphinx-doc/sphinx#12300.