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Fixes #1909

We'll need to release a final greenlet 2.0 and remove the rc qualifiers here before releasing gevent, because pre-release qualifiers are contagious.

Use the non-legacy `setuptools.build_meta` backend.  Prepend the setup
directory to `sys.path` prior to importing modules relative to it.

Fixes #1910

Remove the redundant `wheel` dependency, as it is added by the backend
automatically.  Listing it explicitly in the documentation was
a historical mistake and has been fixed since, see:
https://github.com/pypa/setuptools/commit/f7d30a9529378cf69054b5176249e5457aaf640a

Leaving some notes about needed cleanup.

This first push is just to make sure all the other versions besides 3.11 continue to do their thing.

Fixes #1801.

Fixes #1674

Fixes #1627

Begin trying to install on Travis.

This is because the memory leak tests failed: https://travis-ci.org/github/gevent/gevent/jobs/683782800

The double-dunder name changes broke some things.

Fixes #1599

Fixes #1564.

And stop a fresh greenlet from even running.

Fixes #1473.

Free user-provided resources as soon as the greenlet is cancelled, if it's not running.

Still needs the specific networking test classes added, but all the basics pass for me. Lets see about CI.

The problems with memory fences that were referenced in 7f4e0b01 should be long gone: XCode has moved on, as has libev, and the conditionals in libev also check for __clang__ too, so it wasn't doing anything anymore.

Also update pyproject.toml to reflect what we're actually using version-wise, and add a note about why we use the build backend we do.