swig-py: Make it possilbe to use the notify callbacks with python bindings.

* subversion/bindings/swig/svn_client.i
  (svn_wc_notify_func2_t): Use proper function to be a svn_wc_notify_func2_t.
  (svn_wc_notify_func_t): New.

* subversion/bindings/swig/python/tests/client.py
  (test_update4): New test.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1594794 13f79535-47bb-0310-9956-ffa450edef68