Dropping support for Python 2

[rkty13]

This PR addresses #801. I have gone through all of the files and removed any support for Python 2 that I could find.

Overview of the changes I made:

• Changed all floating point divisions that used a float() cast or had one of the operands as a float (e.g. 1.0/... or 1./....
• Unicode strings are no longer prefixed as such: u''
• Changed all references to superclasses to use the preferred zero-argument format super().
• Removed all imports from the __future__ module.
• Removed some import logic to check if Python's version was 2 or 3.

Some things I could not change:

• Dropping the use of random_choice() and randrange() in https://github.com/Axelrod-Python/Axelrod/blob/master/axelrod/random_.py as noted in #801.
• Due to Multiple Inheritance, it is preferred to use the old-style superclass/direct reference in https://github.com/Axelrod-Python/Axelrod/blob/master/axelrod/match_generator.py#L263.

Overall I think I have caught most of the differences between Python 2 and Python 3. Please let me know if I have missed anything or if I need to add anything else to this PR.

[marcharper]

@rkty13 Can you also update the issue with what is left to do? Even if it's just the last few lines above.

Thanks!

[rkty13]

Done!

[drvinceknight]

What a great contribution! Thanks!