Convert to Python3

[schneiderfelipe]

This PR converts the codebase to Python3. Tests still pass. Some minor changes in level messages are also present.

[yegeniy]

This seems to work (assuming python 3 is the default installed python on your machine).
Keep in mind that this project is likely abandoned by this point.

~/d/g/s/python-warrior> ./bin/pythonwarrior                                                                             15 17:51:18
Welcome to Python Warrior
[1] beginner
[2] intermediate
Choose tower by typing the number: 
1
Enter a name for your warrior: 
yeg
First level has been generated.See the pythonwarrior/yeg-beginner/README for instructions.