Add Python 3.7 support, remove Python 3.3 support

[jonafato]

As of #1733, Python 3.7 is supported, so reflect that in the Trove
classifiers.

As of 2017-09-29, Python 3.3 is end-of-life and no longer receives
updates of any kind (including security fixes), so remove it from the
list of supported versions.

[coveralls]

Coverage remained the same at 92.584% when pulling 4c605ba on jonafato:python-versions into c2a1f4e on spulec:master.

[spulec]

Hmmm, any idea why Travis is failing?

[jonafato]

@spulec I mistakenly removed something required for 2.7 support. I've added that back in and rebased against master, so I expect everything to pass now.

[mikegrima]

@spulec Any issues merging this? We're using 3.7 internally with without moto issues.

[dargueta]

While you're at it here, you could add python_requires to the setuptools.setup() call to make this package unavailable to unsupported Python versions. It won't even show up in pip search. I don't think the classifiers are obeyed by pip.

Unfortunately because you have to skip over 3.0-3.3, it's a little ugly:

setup(
    ...
    python_requires='>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*'
)

[terricain]

I second adding python_requires,

[spulec]

Thanks!