2to3: Apply `imports2` fixer.

No files were changed by this fixer, so add it to the list of
fixers to be skipped by 2to3.

Closes #3181.
MAINT: Append comma to last item tools/py3tool.py skipped fixers.

Makes it easier to put in the next fixer.
Merge pull request #3201 from charris/2to3-apply-imports2-fixer

2to3: Apply `imports2` fixer.