dim: require Python 3

Python 2 has been end-of-life'd, and d79c5784 ("dim: fix handling of 8-bit
non-UTF-8 messages") relies on functions introduced in Python 3.2.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>