Don't hit remote servers when running tests

Use the [responses](https://github.com/openelections/clarify/issues/12) package
to mock HTTP responses in
``TestJurisdiction.test_get_subjurisdictions_state()``.

Made Jurisdiction._scrape_subjurisdiction_summary_path() a class method since
it doesn't access/alter any instance attributes or methods.

Remove ``TestJurisdiction.test_subjurisdictions_state_meta()`` and replace it
with ``TestJurisdiction.test_scrape_subjurisdiction_summary_path()`` since the
``Jurisdiction._scrape_subjurisdiction_summary_path()`` method is the piece
that handles the variation between when the county redirect page redirects with
JavaScript vs. a meta tag.  There's no need to re-test the entire
``Jurisdiction.get_subjurisdictions()`` method.

Remove ``TestJurisdiction.test_get_subjurisdictions_web01()`` and instead test
that the "Web01" portion is stripped from the parsed URL.

Remove unused ``import six``.

Fixes https://github.com/openelections/clarify/issues/12