auth_app function outdated

[meck-gd]

Hi,

The client function auth_app is relying on functionality that Taiga hasn't exposed since v3.1.0 in 2016. Application secrets and cyphered tokens were removed in that version of Taiga - the response simply contains a plain token field now.

This came up because I was looking at usages of the dependency pyjwkest, which is unmaintained. It has a transitive dependency on future, which in turn has an unpatched high severity vulnerability that tripped our pip auditing CI.

I can offer to work on a PR for this, if you'd like.