Update test warnings for Python 3.10.0b2

Switch our PYTHONWARNINGS in tox.ini to use a string match on some
warnings which have become increasingly common for more modules
under Python 3.10.0b2. For those which can't be covered effectively
by matching their messages, get more specific by adding the
exception names to the current module exclusions. Hopefully this
will allow for fairly narrow warning filters while minimizing the
need for future additions, at least to the extent that's possible.