Mark blackd tests with the `blackd` optional marker (#2204)

This is a follow-up of #2203 that uses a pytest marker instead of a bunch of
`skipUnless`.  Similarly to the Python 2 tests, they are running by default and
will crash on an unsuspecting contributor with missing dependencies.  This is
by design, we WANT contributors to test everything.  Unless we actually don't
and then we can run:

  pytest --run-optional=no_blackd

Relatedly, bump required aiohttp to 3.6.0 at least to get rid of expected
failures on Python 3.8 (see 6b5eb7d4).