2to3: Skip itertools_imports fixer.

No files are changed by the itertools_imports fixer so skip it.  What
the fixer does is rename imports of imap, ifilter, and izip to map,
filter, and zip since the latter are iterators in Python 3.

Closes #3234
Merge pull request #3235 from charris/2to3-apply-itertools_imports

2to3: Skip itertools_imports fixer.