Make all library code compatible with both Python 2 and Python 3. Helps
move to modern Python idioms. Can now write for Python 3 (with
workarounds for Python 2) instead of the other way around.
In the future, when it is eventually time to drop Python 2, the library
will be in a better position to remove workarounds
Added a very small comparability module compat.py where required. It
includes definitions for:
- text_type -- A type. str on Python 3. unicode on Python 2.
- string_types -- A tuple. Contains only str on Python 3. Contains str &
unicode on Python 2.
