MAINT: Cleanup some imports involving reduce.

Because reduce has been available in functools since Python 2.6 we
can get rid of the version checks we currently have before we import
it.

Also removes some reduce related skips in tools/py3tool.py. We were
already skipping the reduce fixer so this has no effect other than
cleaning up the code.
Merge pull request #3202 from charris/2to3-reduce-fixups

MAINT: Cleanup some imports involving reduce.