gh-104804: Remove webbrowser.MacOSX class, deprecated in Python 3.11

[hugovk]

Closes #104804.

Remove webbrowser.MacOSX class, deprecated in Python 3.11 in gh-86421.

---

Also fix Sphinx warnings in webbrowser.rst (helps gh-101100):

• I don't think we need to define envvars for PATH or KDEDIR

• We could define .. envvar:: BROWSER right before the first mention, but it looks a little out of place having a BROWSER before the third paragraph. Thoughts?

• I also don't think we need to define the dozen or so Mozilla, Opera etc. classes. They're listed in the table that shows which type name corresponds to each class, and some of them have extra details like how they're instantiated (Mozilla('mozilla'), GenericBrowser('lynx')), so plain literal text makes sense here. Repeating each class below would be a lot of duplication for little benefit, especially for this rather short page.

• https://cpython-previews--104816.org.readthedocs.build/en/104816/library/webbrowser.html

---

📚 Documentation preview 📚: https://cpython-previews--104816.org.readthedocs.build/

• Issue: Remove deprecated webbrowser.MacOSX class #104804

[serhiy-storchaka]

Would not be better to split this PR on two? One for removing, and other for tidying?

[hugovk]

I can do, but they are intertwined: this bit is removed, which otherwise has warnings that would need fixing, or waiting for part 1 to be merged:

.. deprecated-removed:: 3.11 3.13
   :class:`MacOSX` is deprecated, use :class:`MacOSXOSAScript` instead.

Seems easier to do both at once. But either way is fine for me :)

[CAM-Gerlach]

I had a handful of doc-related suggestions. Otherwise, the core change looks good. The removal of the :class: syntax for things that are really code snippits (and not formally documented anyway) makes sense, as does silencing warnings for env vars not explicitly defined by or primarily/exclusively used by Python itself.

Ideally it would be nice to split it into two PRs as @serhiy-storchaka , but as a practical matter given they interlinked, maybe not worth the extra effort.

[vstinner]

LGTM if you fix the typo in the NEWS entry.