Followup for python-requests-toolbelt.

- convert to singlespec packaging

- update to 0.8.0:
- Add ``betamax_parametrized_recorder`` and ``betamax_parametrized_session``
to our list of pytest fixtures so that users will have parametrized cassette
names when writing parametrized tests with our fixtures. (I wonder if I can
mention parametrization a bunch more times so I can say parametrize a lot in
this bullet note.)
- Add ``ValidationError`` and a set of subclasses for each possible validation
error.
- Raise ``InvalidOption`` on unknown cassette options rather than silently
ignoring extra options.
- Raise a subclass of ``ValidationError`` when a particular cassette option is
invalid, rather than silently ignoring the validation failure.
- Fix bug with query string matcher where query-strings without values (e.g.,
``?foo&bar`` as opposed to ``?foo=1&bar=2``) were treated as if there were
no query string.
- Fix issue #108 by effectively copying the items in the match_requests_on
list into the match_options set on a Cassette instance