Add Python3 support

[ryansb]

Hi, I'm looking into this, but as-is this PR breaks all the tests (oh no!) since they mock out the urlopen to intercept the HTTP requests. I'm looking at switching to vcrpy or similar instead of mocking, since that stubs in at a lower level meaning we can leave those conditionals.

The other alternatives are:

• Python 3 only (which might be ok, but would probably make some people sad if they have a Python 2 SDK they use in their custom resources)
• Use six.moves to find all the proper bits (but this would add a dependency meaning folks couldn't paste the lib to the top of a single-file function)

I'm working on the first one (use vcrpy) but first I'll add in #2 to make multi-env tests easier.

[ryansb]

Merged as https://github.com/ryansb/cfn-wrapper-python/commit/0a581aab84b34ce3e3398667782ef9c8433f2670 along with test changes.