Changes include:

- DBusException.get_dbus_message(): In Python 3, the str of the exception will
  already be a unicode, so don't try to decode it unless it's a bytes object
  (a.k.a. 8-bit str in Python 2).
- gobject_service.py: Switch to pygi and rewrite the metaclass instantiation
  code to be portable between Python 2 and Python 3.
- run-test.sh: echo a few more useful environment variables
- test-client.py:
  - Globally replace deprecated assertEquals with assertEqual
  - Globally replace deprecated assert_ with assertTrue
  - Use bytes objects for both 'ay' signatured methods on the server
  - AcceptUnicodeString will return a native unicode, i.e. a str in Python 3
    and a unicode in Python 2.  Python 3 has no `unicode` built-in.
  - Reformat some long lines for debugging.
- test-service.py:
  - Open the log file in 'a' mode for easier tailing.
  - AcceptUnicodeString will return a native unicode, i.e. a str in Python 3
    and a unicode in Python 2.  Python 3 has no `unicode` built-in.
  - reformat some long lines for debugging.
  - Put module-scope code into a main() function and add a bunch of logger
    output for better debugging.  `session_bus` must still be global though.
    Wrap main() in a bit try/except to log all top-level exceptions.

Specifically, DBusException and its subclasses no longer have the remote
traceback by default (although subclasses can turn it back on again
by setting include_traceback = True, and the various "programmer error"
subclasses of DBusException do have this set).

Hopefully this will stop people thinking it's a dbus-python or
telepathy-python bug when a D-Bus API like Telepathy deliberately raises an
error (and so dbus-python or telepathy-python is visible in the traceback).

Add document API_CHANGES.txt listing visible API changes.
Add more test cases, for low-level Python <-> D-Bus type mappings.
Amend existing test cases to cope with the API changes.

Ditto for dbus_glib_bindings.
Remove dbus.pth - should no longer be needed after this change.