Cleanup tox configuration

There were issues with dependencies and tox environments under Python 3
so refactored the tox configuration to be more consistent and clearer.

- Moved travis to default to Python 3 for linting and tests.
- Fixed missing mock for cairo in sphinx config.
- Collated the base deps sections to improve readability.
- Added PYTEST_ADDOPTS env to override pytest verbosity in just tox
  tests as this was a common option being used.
- Renamed env 'testcoverage' to the more concise 'coverage' and moved
  html creation under single env as handy to have this output as well
  as report.
- Cleaned up the isort config for gtk3.
- Added `bad-continuation` to pylint config as conflcts with black
  formatting.
- Fix isort issue with bbfreeze script. This will likely be removed
  in future so just skip sorting it.

.pylintrc

@@ -69,7 +69,7 @@ confidence=
 # Arranged by category and use symbolic names instead of ids.
 disable=
 # Convention
-    missing-docstring, invalid-name,
+    missing-docstring, invalid-name, bad-continuation,
 # Error
     no-member, no-name-in-module,
 # Information