Group some tests in the same process

Fixes #1581.

This was exposed by running test__os and test__queue in the same process in that order. test__os watches children,
and test__queue has a test that expects a LoopExit. It wasn't raised because the child watcher (really the signal watcher)
was referenced, keeping the loop running.

Fixes #1580

This almost cuts the test time in half on my machine.

it wasn't in the matrix there.

Make it possible to monkey-patch contextvars of the non-standard back…

Make it possible to monkey-patch contextvars of the non-standard backport is installed on Python < 3.7.

Fixes #1572.

Refactor known_failures.py to use a compact DSL instead of a maze of nested branches

Part of #1367

Update PyPy to 7.3.1 on Windows

They take special linking (unlike CPython) because the PyPy maintainers don't want you using them. That leaves us without a way to add memory pressure from C on PyPy.

So back to 7.3.1.

Bug filed https://foss.heptapod.net/pypy/pypy/issues/3213

It looks like that got interpreted as an error?
https://ci.appveyor.com/project/denik/gevent/builds/32221986/job/2hveawuceoxsbdxj

https://ci.appveyor.com/project/denik/gevent/builds/32220510/job/wsqay6l44eygunvf#L499
       Creating library build\temp.win32-2.7\Release\build\temp.win32-2.7\Release\_corecffi.pypy-73.lib and object build\temp.win32-2.7\Release\build\temp.win32-2.7\Release\_corecffi.pypy-73.exp
500    gevent.libuv._corecffi.obj : error LNK2019: unresolved external symbol __imp__PyPyObject_Free referenced in function _gevent_realloc
501    gevent.libuv._corecffi.obj : error LNK2019: unresolved external symbol __imp__PyPyGILState_Ensure referenced in function _gevent_realloc
502    gevent.libuv._corecffi.obj : error LNK2019: unresolved external symbol __imp__PyPyGILState_Release referenced in function _gevent_realloc
503    gevent.libuv._corecffi.obj : error LNK2019: unresolved external symbol __imp__PyPyObject_Realloc referenced in function _gevent_realloc
504    build\lib.win32-2.7\gevent\libuv\_corecffi.pypy-73.pyd : fatal error LNK1120: 4 unresolved externals
505    error: command 'C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\BIN\link.exe' failed with exit status 1120
506    ---------

Refs #1569

Refs #1555

Let testing work in manylinux when test.support isn't installed.

If test.support can't be imported many of the stdlib tests cant run either.

Refs #1555

Make monkey-patching properly remove select.epoll and select.kqueue.

A missing comma led to string concatenation and a silent bug. I'm surprised this wasn't really tested anywhere.

Fixes #1570

Fixes #1564.