Fixes #15446

setuptools has broken how editable installs work. It seems like most
other build backends continue to use static pth files, so let's just
switch to another one.

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

pytest 7.4.0 was just released, and our CI on `master` is very red as a
result. Looks like we're using a private API that we shouldn't be:
#15501 (comment). For
now let's just pin to pytest <7.4.0.

https://pypi.org/project/pytest/7.4.0/

Makes the HTML builder generate a _refs.html file that redirects
(global) refs (when provided as a hash i.e. `_refs.html#some-ref`) to
the appropriate document.

This allows tools (e.g. #15431) to link to well-known refs.

Fixes #15431

After this PR one will be able to easily find documentation for given
error code using
`https://mypy.readthedocs.io/en/stable/_refs.html#code-{code as reported
by mypy}`, for example
`https://mypy.readthedocs.io/en/stable/_refs.html#code-attr-defined`.

I initially included an error (removed allowlisted error code that is
not covered), to verify it will be caught.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Fixes #15489