Fixes #1588 and fixes #1589

Fixes #1567, fixes #1562, fixes #1559 and fixes #1566

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