Added Python 3 support.

This patch adds support to python 3. It's basically a pycharm inspection + autofix, and since there are no test, running  ```python play.py extermination 80x20 sniper,troll -n 50 -m to_the_closet``` for some rounds without crashing.

The things that were not automated fixes were:

1. The extraction of ```dead_things()``` to return a filtered list. This avoids removing keys from the dictionary being iterated.
2. The py3/py2 if in ```import_map()```. This is nasty and needs to be cleaned up, but it works.

Closes issue #1