Return tokens as string not bytes

[jpadilla]

Extracted from #470

[mato2000]

Hello, I would like to know why tokens was changed to be string instead of bytes. It seems to me that it is more suitable to be bytes. Thanks

[personalcomputer]

Hi @jpadilla, thanks for all your work on PyJWT. Just as a quick bit a feedback, I want to let you know that this return type
change from bytes to str is a breaking change. Updating to 2.0.0 caused all our code interacting with encode() to fail, as our code has been expecting bytes in return. It was hard to identify what happened as the release notes had listed this item embedded alongside all the other internal changes. It would be helpful that when publishing a new release, to prominently mention breaking API changes in the release notes, so that we can easily see them and update our code to support the changed interface when updating the library. Not required though, just thought I would let you know. Hey whoops - Looks like you've fixed this in the CHANGELOG.md. Just was missing in the notes in the Github Releases page. My bad that we missed to read the CHANGELOG.md. https://github.com/jpadilla/pyjwt/pull/584/files