Add trove classifiers to add metadata to JSON API

[cooperlees]

Hi,

Many thanks for the project!

How do you feel about adding trove classifiers to your setup.py about Python versions so API calls can be used to find that information?
e.g. My build system checks https://pypi.org/pypi/aiosocks/json to see if it's Python 3 only or not.

PR coming.

[nibrag]

added