Prefer importing math.gcd, for Python 3.5
Prefer importing math.gcd, for Python 3.5

Python 3.5 moves gcd() from fractions to math. There is a compatibility
alias, but importing it raises a DeprecationWarning on Python 3.5.

abalkin/dateutil,pganssle/dateutil-test-codecov,pganssle/dateutil-test-codecov,abalkin/dateutil