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, 3 deletions

diff --git a/setup.cfg b/setup.cfg
index 4f15bcef5..4651662e3 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -19,10 +19,8 @@ frameworks = CoreFoundation, Foundation, AppKit
 [isort]
 known_standard_library = future_builtins
 known_third_party =
-# Ignore Windows specific modules.
-    bbfreeze, win32verstamp,
 # Ignore gtk modules, primarily for tox testing.
-    pygtk, gtk, gobject, gtk.gdk, pango, cairo, pangocairo, gi
+    cairo, gi,
 # Ignore other module dependencies for pre-commit isort.
     twisted, OpenSSL, pytest, recommonmark, chardet, pkg_resources, zope, mock,
     sphinx