Fix auth_app for modern Taiga versions (#227)

[meck-gd]

Description

• Fix auth_app() to (only) work with Taiga 3.1.0+ (released in 2016).
• The method returns the token now instead of setting it up in the API client. This is because it's not really meant to be used like auth() - rather, it only works once per token because the Taiga backend invalidates the auth_code during the request. From what I can tell, applications are supposed to store and re-use the token. It doesn't have an expiration, unlike Bearer tokens.
• Dependency pyjwkest is no longer required.

References

Fixes #227 Due to removal of dependency, also fixes #189

Checklist

• [x] I have read the contribution guide
• [x] Code lint checked via inv lint
• [x] changes file included (see docs)
• [ ] Usage documentation added in case of new features
• [ ] Tests added