Fix tox config for v4

[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 #837 (6e308fc) into master (b3c93f5) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #837      +/-   ##
==========================================
+ Coverage   63.81%   63.82%   +0.01%     
==========================================
  Files          64       64              
  Lines       11882    11880       -2     
  Branches     2158     2158              
==========================================
  Hits         7583     7583              
+ Misses       3695     3694       -1     
+ Partials      604      603       -1     

Impacted Files	Coverage Δ
src/oic/extension/sts.py	0.00% <0.00%> (ø)
src/oic/oauth2/message.py	74.25% <0.00%> (+0.10%)	⬆️

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[schlenk]

LGTM