Clarify supported versions of Python 3

[chrish42]

Change the wording to explicitly mention that Python 3.3 and 3.4 are supported (instead of saying "3.3+"), as these are the versions that are tested in the tox.ini file.

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[chrish42]

I signed the CLA.

[googlebot]

CLAs look good, thanks!

[chrish42]

Haha. Sure! I can even update the tox.ini file to test on 3.5 too, if you want. :-)

A bit more seriously, testing on 3.5 would be really nice, because it's much more interesting than 3.4 for both scientific computing (matrix multiply operator) and writing server software (async & await).

[theacodes]

Thanks, @chrish42

[theacodes]

I can even update the tox.ini file to test on 3.5 too, if you want. :-)

@chrish42 be my guest. It's been on my list but I've been busy with google-auth and haven't had time to circle back to this library.