Fix auth_app for modern Taiga versions (#227)

[heck-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

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

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.07%. Comparing base (b258c1a) to head (5a4f5a8).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
taiga/client.py	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #229      +/-   ##
==========================================
+ Coverage   96.47%   97.07%   +0.60%     
==========================================
  Files           8        8              
  Lines         936      923      -13     
  Branches       67       64       -3     
==========================================
- Hits          903      896       -7     
+ Misses         19       16       -3     
+ Partials       14       11       -3     

Flag	Coverage Δ
unittests	97.07% <75.00%> (+0.60%)	⬆️

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[coveralls]

coverage: 98.267% (+0.3%) from 97.97%
when pulling 5a4f5a8 on GDATAAdvancedAnalytics:bugfix/227-fix-app-auth
into b258c1a on nephila:master.

[protoroto]

@heck-gd Thanks for this! I'll ping @yakky to have a look at this as soon as possibile, I added a small comment because I'm not entirely sure about what should auth_app method return.