Support Python 3

[amotl]

2020: Python 2 is dead. Long live Python 3!

@frankheider already chimed in (thanks again!) to add basic Python 3 support within [1]. However, this is just a start. The codebase will have to be converted completely and the functionality has to be checked for any quirks eventually coming from this transition.

[1] https://github.com/frankheider/ip-navigator-py3.6/commits/python3

[amotl]

With #49, we are adding a test harness in order to cover the migration. On another matter, we also just found back loucash/mongodb_gridfs_beaker#1, which will be needed along the way.

[amotl]

Hi again,

coming from GH-68, @papoteur-mga just submitted GH-70, in order to conclude the migration to Python 3. Thank you so much!

However, when looking at the changes since version 0.169.3 (2019-11-01), I think there should be another release beforehand, still on Python 2.

With kind regards,
Andreas.