Groom supported Python versions and declare in setup.py metadata

[aoberoi]

Description

It seems like a bunch of unsupported Python versions are running in the tests. I think it should be the same as those for slackclient, which are py{27,34,35,36}. This seems to be the cause of failures in open PRs.

Also, declaring the specific versions that are supported in the setup.py metadata, as done in slackclient would make it easier for users and machines to understand if their system is compatible or not.

Requirements

• [x] I've read and understood the Contributing guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [x] I've searched for any related issues and avoided creating a duplicate issue.

[seratch]

We've fixed https://github.com/slackapi/python-slack-events-api/issues/53 by merging https://github.com/slackapi/python-slack-events-api/pull/68

As this package depends on Flask, it should align with Flask's policy.

As of Flask 1.1.2, it supports the following versions:

Requires: Python >=2.7, !=3.0., !=3.1., !=3.2., !=3.3., !=3.4.*

I will make a pull request to set the supported versions.