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
MAINT: Remove unneeded version checks.

Now that only Python versions 2.6-2.7 and 3.2-3.3 are supported
some version checks are no longer needed. This patch removes them
so as to clean up the code.
MAINT: Remove the tools/py3tool.py file.

The tools/py3tool.py file was responsible for running 2to3. Now that
2to3 is no longer run it is not needed and can be removed.
Merge pull request #3269 from charris/maint-remove-py3tool-file

MAINT: Remove the tools/py3tool.py file.
Merge pull request #3268 from charris/maint-remove-unneeded-version-checks

MAINT: Remove unneeded version checks.