Add enum34 to doc requirements (for import).
Add enum34 to doc requirements (for import).

dopplershift/MetPy,ShawnMurd/MetPy,ahaberlie/MetPy,ahaberlie/MetPy,jrleeman/MetPy,dopplershift/MetPy,Unidata/MetPy,deeplycloudy/MetPy,ahill818/MetPy,Unidata/MetPy,jrleeman/MetPy