Drop Python <= 3.5 support

[thinkingserious]

It is on our backlog to drop support for Python <= 3.5 as they are EOL. This issue exists to help identify potential issues an to gather general feedback. Please feel free to comment with any questions or concerns.

Related Issue: #556

[wavenator]

I am totally in favor of this movement. There are so many reasons to make that move, and I think the strongest one is being aligned with the community. Every day more and more packages drop their Python2 support.

[hugorodgerbrown]

Coming here from the #556 issue - having to support 2/3 and pinning to an old PyJWT is causing us upgrade isses.

[ryanhiebert]

In favor as well. Looking to update PyJWT as well, and I think that dropping Python 2 is perfectly reasonable. If folks need to stay on Python 2 for a bit longer, the old versions of this library don't go away, so I don't think we need to worry about that, really.

[childish-sambino]

Updated title and description. We will be looking to major version roll this library in the coming weeks and drop support for Python <= 3.5 and migrate to PyJWT 2. Comms will go out to customers using such versions of Python and a recent release of this library to notify them of the breaking change should they choose to upgrade.

[shwetha-manvinkurke]

Closed in #560