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.

1 files changed, 1 insertions, 0 deletions

diff --git a/requirements-tests.txt b/requirements-tests.txt
index 841b68fa4..e499bf989 100644
--- a/requirements-tests.txt
+++ b/requirements-tests.txt
@@ -2,6 +2,7 @@ pytest
 pytest-twisted
 pytest-cov
 mock
+pre-commit
 flake8
 flake8-quotes
 flake8-isort