CLN: fix all flake8 warnings in pandas/tseries
Looks good from a quick glance! (and for the diffs I could see)
 Cool, I can merge after Travis passes if we're all good
 If someone can +1 this and also review #12082 I can merge both of those
 I fixed it! 
 lgtm otherwise
 @jreback I nixed the resample.py diff, will merge when the build passes
 Cherry-picked in 1acdb31d9348f70f1cc77790fbc7bee568366cdd
 merge away
