Fixed errors caused by PyJWT 2.0.x update.

[gwynwilkinson]

This sample (and accompanying blog at https://medium.com/@ratrosy/building-a-basic-authorization-server-using-authorization-code-flow-c06866859fb1) is really good for understanding OAuth 2.0 and how JWT are used.

Unfortunately the existing code for the Access Code flow causes some errors, I assume due to changes in PyJWT 2.0.0 which were simple to spot and fix (as you can see, the commits are very small).

I will continue working through the other OAuth 2.0 flows, and will commit changes to those if I come across the same issues. Thanks again for your work on this!

G