Update platform_strategy.py to support Python 2.x

[neosergio]

metaclass change in Python 3.x, but there are lot of projects still using Python 2.x, so for compatibility is good to change metaclass syntax

[philipplaeubli]

Hey there, many thanks! As I understand though, this won't run on Python 3, see http://python-future.org/compatible_idioms.html#metaclasses

So we should use this:

class PlatformStrategy(with_metaclass(ABCMeta)):
    def __init__(self, platform_application):
...

Could you update this and add six or future to the requirements.txt?

[neosergio]

Thank you for your feedback @philipplaeubli, I changed the pull request as you recommend

[alexgv14]

this would be sweet thanks @neosergio can't wait for it to get merged.

[philipplaeubli]

Dear @neosergio, thank you very much. There are some other compatibility issues i've stumbled upon while testing your changes. I'll merge your branch, clean up some other issues and release a new version on pypi.

[neosergio]

Thank you @philipplaeubli