fixed #1042 -- updates an import to use six from the vendored package

Document -s to run single test

Fixes https://github.com/pypa/setuptools/issues/1032

Tox doc comments

This will help get new contributors started, if they're just used to
running "tox".

Use a different method to lookup base classes on Jython

Make _get_mro private; Swap logic to put preferred behavior at top level; Update docstring to reference issue.

Jython seems to implement inspect.getmro differently, which causes
any classes with the same name as a class lower in the MRO not to
be returned.

This patch offloads the MRO lookup to a separate function, which
implements different logic for Jython only.

Ref #1024

Rewrite tests to test the actual matching rather than making assertions about the regular expressions. Fixes #1015.