MNT: Add python_requires to setup.py
MNT: Add python_requires to setup.py

This will be more important once we drop 2.7.

jrleeman/MetPy,dopplershift/MetPy,dopplershift/MetPy,jrleeman/MetPy,ahaberlie/MetPy,ahaberlie/MetPy,ShawnMurd/MetPy,Unidata/MetPy,Unidata/MetPy