Drop python 3.6

[tpazderka]

• 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 #810 (2cbbfa4) into master (a59f34c) will increase coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 2cbbfa4 differs from pull request most recent head 3f88182. Consider uploading reports for the commit 3f88182 to get more accurate results

@@           Coverage Diff           @@
##           master     #810   +/-   ##
=======================================
  Coverage   63.70%   63.71%           
=======================================
  Files          64       64           
  Lines       11846    11849    +3     
  Branches     2401     2410    +9     
=======================================
+ Hits         7547     7550    +3     
- Misses       3701     3702    +1     
+ Partials      598      597    -1     

Impacted Files	Coverage Δ
src/oic/utils/authn/user.py	63.41% <0.00%> (-0.82%)	⬇️
src/oic/utils/http_util.py	69.25% <0.00%> (-0.58%)	⬇️
src/oic/utils/client_management.py	50.37% <0.00%> (-0.38%)	⬇️
src/oic/extension/sts.py	0.00% <0.00%> (ø)
src/oic/utils/keyio.py	71.56% <0.00%> (+0.13%)	⬆️
src/oic/oauth2/message.py	74.14% <0.00%> (+0.42%)	⬆️
src/oic/utils/time_util.py	91.71% <0.00%> (+1.33%)	⬆️

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[schlenk]

LGTM.