CLN: Fix compile time warnings
- [x] passes `git diff upstream/master | flake8 --diff`

This commit suppresses these warnings

warning: implicit conversion from enumeration type\
'NPY_DATETIMEUNIT' to different enumeration type\
'PANDAS_DATETIMEUNIT' [-Wenum-conversion]

will have to test on windows
 Can I ask you to check on windows ? 😃 
 cos we have appveyor.yml, u can run it on AppVeyor. though @jreback may have more comprehensive environment.
 Can you rerun CI?
 thanks!
