Unicode strings aren't bytestrings, so there's no obvious meaning for
the byte value of a Unicode string of length 1.

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.

Fix another except syntax problem.

object paths in Python3, as per Simon's review comments.

- Disallow appending unicode objects with 'y' (bytes) signatures.  This now
  requires either a bytes object or an integer.  Update the tests to reflect
- this change.
- Fix broken __all__ in Python 3.

2, since there are still some unconditional PyInt calls, which are not valid
in Python 3.  However, it lays the framework for conditionalizing on Python 3
and using only PyLong in that case.  Where it doesn't matter, PyLong is used
unconditionally.