TST: non-strict xfail for period test
This was XPASSing on the MacPython builds:
https://travis-ci.org/MacPython/pandas-wheels/jobs/574706922
LGTM.

Darn though; I'll look at the macpython build and see if the condition can be made more strict If it's Python 3.5 only it may not be worth digging too much. I'm not sure how long we'll support it once 3.8 is out.