Declare that we support Python 3

Note that this should not land until we figure out why `tox -e py33` doesn't
actually run any tests.

Change-Id: Id4e080cfbfca45b3bf7ed81b03a31414495a1348
Provide a link to the documentation to the README

Change-Id: I99a46860ffff93452904d24504a4ccc58e8e64c6
Merge "Declare that we support Python 3"
Merge "Provide a link to the documentation to the README"