added 'dateutil.tz' subpackage to setup.py
added 'dateutil.tz' subpackage to setup.py

(distutils won't find it on its own: https://docs.python.org/3/distutils/setupscript.html#listing-whole-packages )

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