switch to implicit namespaces

Remove explicit namespace package handling. See
https://peps.python.org/pep-0420/

Fixes #32

Author: dhellmann

setup.cfg

@@ -24,8 +24,6 @@ zip_safe = False
 [files]
 packages =
     virtualenvwrapper
-namespace_packages =
-    virtualenvwrapper
 scripts =
     virtualenvwrapper.sh
     virtualenvwrapper_lazy.sh