Fix #1733

Because thread.get_ident() returns types of different signedness on Py2/Py3,
and some, possibly 32-bit ILP32, Py3 systems return values that won't fit in a long.

Cython conditional compilation doesnt work because we can only publish one source distribution.

Fixes #1733

Make Greenlets context managers to handle their lifetime.

Fixes #1324

docs: fix simple typo, empoyed -> employed

There is a small typo in src/gevent/tests/test__pywsgi.py.

Should read `employed` rather than `empoyed`.

Make loops automatically break when their owning hub is destroyed.

Tweak wording about Fedora package managers in docs/development/installing_from_source.rst. [skip ci]

Fixes #1689 with thanks to Fabian Affolter.

Supercedes and Fixes #1695; thanks to Anatoli Babenia.

And stop running callbacks.

This fixes #1686 and fixes #1669.

Refactor _socket2/_socket3 to move more truly common methods into SocketMixin

Move socket objects fully to __slots__.

Fixes #1724

Make gevent.pywsgi stop dealing with chunks when the connection is being upgraded

Let the application have full control over input and output.

Fixes #1712.

Per https://github.com/actions/setup-python/issues/171#issuecomment-747917974

Install thread profiling/tracing hooks in ThreadPool worker threads while the task runs

Try killing the pool to see if that buys us what we need.

Fixes #1670

Fixes #1719

Fix #1663 by more gracefully handling non-type arguments.

The documentation specifically says that kill, killall, etc, all take a a type, but
allow passing exception instances.

Also allow passing arbitrary objects; this is also not documented, and not recommended.
The result will be raising a BaseException in the greenlet. That should be better than
silently printing something to stderr, which is what happened before.