ENH: Skip already applied fixers when running 2to3.

This is done in `tools/py3tool.py` by providing a list of fixers that
is passed in the call to 2to3 with with the `-x` option that tells
2to3 to skip them.

Closes #3113
Merge pull request #3115 from charris/2to3-skip-fixers

ENH: Skip already applied fixers when running 2to3.