Fix mypy errors with no_implicit_optional=True

[schlenk]

• Any changes relevant to users are recorded in the CHANGELOG.md.
• The documentation has been updated, if necessary.
• New code is annotated.
• Changes are covered by tests.

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[codecov-commenter]

Codecov Report

Merging #832 (c1f79da) into master (954ee64) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #832   +/-   ##
=======================================
  Coverage   63.76%   63.77%           
=======================================
  Files          64       64           
  Lines       11882    11883    +1     
  Branches     2159     2159           
=======================================
+ Hits         7577     7578    +1     
  Misses       3700     3700           
  Partials      605      605           

Impacted Files	Coverage Δ
src/oic/oauth2/provider.py	70.70% <ø> (ø)
src/oic/oic/__init__.py	64.99% <ø> (ø)
src/oic/oic/provider.py	74.07% <ø> (ø)
src/oic/oauth2/__init__.py	69.95% <100.00%> (ø)
src/oic/oauth2/base.py	55.40% <100.00%> (+0.61%)	⬆️

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