ag -l Copyright | xargs sed -i \
        "s/\(.*copyright (C) [0-9]\+\)\(-[0-9]\+\)\?\(.*\)/\1-$(date +%Y)\3/I"

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.

Close #387