Remove support for installed eggs distributions

[sbidoul]

I could not find the deprecation issue for this, but this was scheduled for removal in 23.3.

closes #12330

[sbidoul]

This is also not super urgent and could be deferred.

[pradyunsg]

One news fragment change, but rest seems fine to me!

[sbidoul]

Thinking further about this, I'd propose to defer the removal for one year at least because

• we are not nearly close to removing pkg_resources
• the removal of the setup.py install code path was not so long ago and users may unknowingly have installed eggs if they did not have wheel installed, which was also the default for python -m venv
• if we remove discovery, uninstallation must be done manually and this may generate a lot of support issues

[uranusjr]

It’s also only until recently related issues started popping up (#12297 and IIRC there’s another one I closed). Either distributions take a lot of time to upgrade to Python 3.11, or people take a lot of time to upgrade to it. In either case, more time is likely a good idea since removing the warning would simply cause dangling files.

[sbidoul]

The related issue as gone_in="25.1", so I changed the milestone accordingly