Add Python 3.6 trove classifier to document support

[jdufresne]

Python 3.6 support was added in 7a1f48e.

Document general Python 2 support as well.

[cclauss]

My sense is that we should do two things:

1. Drop support for versions of CPython that are EOL.
   • https://docs.python.org/devguide/index.html#branchstatus
2. Be explicit (not implicit) about the versions of CPython that we support.

    "Programming Language :: Python :: 2.7",
    "Programming Language :: Python :: 3.4",
    "Programming Language :: Python :: 3.5",
    "Programming Language :: Python :: 3.6",
    "Programming Language :: Python :: 3.7",

[cclauss]

Or maybe shadow requests https://github.com/requests/requests/blob/master/setup.py#L90

[jdufresne]

Drop support for versions of CPython that are EOL.

Sure thing. I'll approach this in a different PR as it will be a much bigger change.

Be explicit (not implicit) about the versions of CPython that we support.

Is this a request for a change? This PR is being explicit. It documents explicit Python 3.6 support. I didn't include Python 3.7 as this repo is not yet testing against Python 3.7. Would you like my to add the 3.7 classifier as well?

[jdufresne]

Looks like 3.7 is being handled in #366

[jdufresne]

Drop support for versions of CPython that are EOL.

Started this process in #371

[jmadler]

That approach makes sense to me. I've merged in #367 and will merge in #371 to adjust the Travis runner and code to reflect that support.

This and #366 both cover .travis.yml / setup.py changes for 3.6/3.7, but neither seem to fix the existing test failures. Is that something you two might be interested in making progress on? I think once we have tests working and #368 merged in, we can do a release cut and start looking at 1.0 after we've burned down more of our bug balance.

@cclauss re implicit/explicit do you mean to say that we should avoid the ":: 2" and ":: 3" trove classifiers?

[jdufresne]

The ":: 2" and ":: 3" document that Python 2 and Python 3 support exists in some capacity. I think they should remain.

neither seem to fix the existing test failures.

I can start investigating these. Do you know why Travis CI is no longer running?

[jdufresne]

I rebased to resolve merge conflicts.