Add threading CI job

[Liam-DeVoe]

This won't work until pytest-run-parallel releases v0.6.0 with support for running hypothesis tests. Hopefully that's soon: Quansight-Labs/pytest-run-parallel#96 (comment)

[Zac-HD]

• I'd like to have an issue to remove many of these threading skips when we can fix the tmp_path(_factory) fixture problem. Maybe have a separate skip decorator for these?
• Similarly, let's make the time-related skips more precise with an @skipif_time_unpatched? It can be defined as skipif_time_unpatched = skipif_threading, but at call sites it's nice to know why we've skipped in that bit of extra detail.

[Liam-DeVoe]

Probably we should generalize crosshair's Why to share skip infrastructure among all cases (emscripten, crosshair, threading, others). We're testing enough of a variety that getting good static feedback here, in a consistent way, seems nice. (not today, though..)

[Liam-DeVoe]

I'd like to have an issue to remove many of these threading skips when we can fix the tmp_path(_factory) fixture problem. Maybe have a separate skip decorator for these?

Which fixture problem is this? If it's just "concurrent writes to the same temp path under threading", I actually think it's better to leave tests like this skipped than to change to tmp_path / threading.get_ident(). Changing this test to run under threading wouldn't test anything meaningful.

@skipif_threading
def test_two_directory_databases_can_interact(tmp_path):
    db1 = DirectoryBasedExampleDatabase(tmp_path)
    db2 = DirectoryBasedExampleDatabase(tmp_path)
    db1.save(b"foo", b"bar")
    assert list(db2.fetch(b"foo")) == [b"bar"]
    db2.save(b"foo", b"bar")
    db2.save(b"foo", b"baz")
    assert sorted(db1.fetch(b"foo")) == [b"bar", b"baz"]

[Zac-HD]

LGTM, merge when you're happy.