Update PyJWT & loosen dependency contraint

[k4nar]

This PR updates PyJWT to the latest version and, more importantly, loosen the dependency constraint on PyJWT at it is using Semantic Versioning.

This should allow any project using segment-analytics-python to update their dependencies 🙏 .

Supersedes #475

[wo0dyn]

Thanks for the update! 🙌🏼

[DavidCain]

Thank you! PyJWT now has a 2.9.0 release -- is there any chance we could change the version that this library depends on to just be any 2.* version?

[k4nar]

@DavidCain : "PyJWT~=2.8" means that any future release of PyJWT 2.x are accepted, so I think it’s ok :) .