ENH: Add conftest for pytest.
ENH: Add conftest for pytest.

This add a header line to the tests that prints out the versions of the
core dependencies when running the tests.

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