Rename Python2CountEqualMixin -> Python2AssertMixin

* Update Python2AssertMixin docstring
Add `assertRegex` to `Python2AssertMixin`

* Allow using python 3 `assertRegex` instead of python 2
  `assertRegexpMatches`
Add Python2AssertMixin to CHANGELOG
Use consistent test function naming
Use real unittest.TestCase in test_compat.py
Add requires_python3, requires_python2 decorators
Add python2 TestCase work-around
Add docstrings to Python2AssertMixin tests
Simplify tests for Python2AssertMixin
Merge pull request #32 from incuna/python-2-assert-mixin

Rename Python2CountEqualMixin to Python2AssertMixin