Keep tests working w/ upcoming aiohttp 4.0.0 (#2974)

aiohttp.test_utils.unittest_run_loop was deprecated since aiohttp 3.8
and aiohttp 4 (which isn't a thing quite yet) removes it. To maintain
compatibility with the full range of versions we declare to support,
test_blackd.py will now define a no-op replacement if it can't be
imported.

Also, mypy is painfully slow to use without a cache, let's reenable it.