A few cases of gethostbyaddr don't match the exception.

Share quite a bit more code between the ares and dnspython resolver.

Addresses #1459

The c-ares resolver comes even closer to the system resolver.

Tests pass locally on a manylinux docker image. So take it out of the known flaky list for now, see how that does.

Does not yet handle canonical names, but this doesn't result in test failures.

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.