Drop support for end-of-life Python versions

[judahrand]

trafficstars

Python versions below 3.5 are now considered end-of-life with 2.7 support being dropped on 1st January 2020 and 3.4 on 3rd March 2019. (https://devguide.python.org/devcycle/#end-of-life-branches)

It also seems that the previously downloaded .tar.gz for Python 3.2 has disappeared resulting in test failure.

[hugovk]

How about also adding some Trove classifiers?

This would then also fix https://github.com/tomasbasham/ratelimit/issues/33.

For example:

        "Programming Language :: Python :: 3",
        "Programming Language :: Python :: 3.5",
        "Programming Language :: Python :: 3.6",
        "Programming Language :: Python :: 3.7",
        "Programming Language :: Python :: 3.8",
        "Programming Language :: Python :: 3 :: Only",