DEPR: Removing previously deprecated datetools module (#6581)
- [X] related to #6581
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

lgtm. though not running on travis for some reason. cc @TomAugspurger 

maybe need to login to travis and turn on pandas-dev for your fork? @datapythonista needs rebase, hopefully CI problem will fix itself. can you rebase, for some reason travis is not running either - not sure why thanks @datapythonista 