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 )
diff --git a/setup.py b/setup.py
index e9185ae..1268965 100644
--- a/setup.py
+++ b/setup.py
@@ -28,7 +28,7 @@
 The dateutil module provides powerful extensions to the
 datetime module available in the Python standard library.
 """,
-      packages=["dateutil", "dateutil.zoneinfo"],
+      packages=["dateutil", "dateutil.zoneinfo", "dateutil.tz"],
       package_data={"dateutil.zoneinfo": ["dateutil-zoneinfo.tar.gz"]},
       zip_safe=True,
       requires=["six"],