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.
97e7d95dd3