PyCalCal for Python 3

[pgr123]

I think I succeeded in porting PyCalCal for Python 3 with a version which works for Python 2 and Python 3.

So, if somebody is interested to test that version, thank you to let my a message.
Good weekend.
pgr123

[rn123]

Had to make one small change to get it working with my Python 3.6:

-    from __builtin__ import round
+    from builtins import round

[rn123]

Submitted a pull request.