2to3: Stop using 2to3 and remove the tools/py3tool.py file.

The same code base now supports python versions 2.6-2.7 and 3.2-3.3.

Closes #3247.
DOC: Document that the 2to3 fixer is no longer used.
MAINT: Use from future_builtins zip and map.

For Python versions 2.6 and 2.7 the iterator forms of zip and map
can be imported from future_builtins. That is done here so as to
avoid using itertools.{izip, imap}.
Merge pull request #3266 from charris/2to3-remove-2to3-fixer

2to3 remove 2to3 fixer