Allow OpenID Client registration with JWT

[JustKiddingCode]

When register an OpenID Client,
registration tokens that are valid JWTs are now passed through to the
OpenID Provider. Every other string is still base64 encoded.
This closes #718

• Any changes relevant to users are recorded in the CHANGELOG.md.
• The documentation has been updated, if necessary.
  Don't know if the previous two are applicable.
• New code is annotated.
• Changes are covered by tests.

---

[tpazderka]

Please add entry in CHANGELOG.

Also the quality checks are failing. Please run make isort and make blacken on the code. That should fix most of the issues.

[tpazderka]

Thank you for the contribution!

[codecov-io]

Codecov Report

Merging #719 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #719      +/-   ##
==========================================
+ Coverage   63.77%   63.79%   +0.02%     
==========================================
  Files          63       63              
  Lines       11689    11696       +7     
  Branches     2056     2056              
==========================================
+ Hits         7455     7462       +7     
  Misses       3662     3662              
  Partials      572      572

Impacted Files	Coverage Δ
src/oic/oic/__init__.py	66.6% <100%> (+0.22%)	⬆️

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