2to3: Skip `future` fixer.

The `future` fixer removes the `from __future__ import ...` statements.
That is fine for Python 3, but we need to keep that statement if we are
shooting for a common code base for both Python 2 and Python 3.
Merge pull request #3223 from charris/2to3-skip-future-fixer

2to3: Skip `future` fixer.