Add Python 3 trove classifiers

[jmbowman]

Although pysrt works with Python 3, it's not advertised as doing so. This PR adds the appropriate trove classifiers so that caniusepython3 and other tools correctly show its Python 3 support status. I also added tox support to make it easier to test compatibility with the various Python versions locally (if you have them installed):

$ pip install tox
$ tox

[coveralls]

Coverage remained the same at 71.753% when pulling 3bbad74a74f3f127eab47df2c091ae63c9597969 on jmbowman:classifiers into b6812e67c8998661cf8658648ab4c89a296299a5 on byroot:master.

[byroot]

Thanks!