rename post_processors submodule to postprocessors
Update supported Python versions in tools/test_pr
Merge pull request #4075 from takluyver/test-pr-pyversions

Update supported Python versions in tools/test_pr
Also renamed local variables in nbconvertapp
Merge pull request #1 from jdfreder/rename_postprocessors

Also renamed local variables in nbconvertapp
Merge pull request #4073 from jakevdp/rename_postprocessors

rename ``post_processors`` submodule to ``postprocessors``