Fix verification of JWT using verify keys

[tpazderka]

Close #508

• Any changes relevant to users are recorded in the CHANGELOG.md.
• The documentation has been updated, if necessary.

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[codecov-io]

Codecov Report

Merging #511 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #511      +/-   ##
=========================================
+ Coverage   59.18%   59.2%   +0.01%     
=========================================
  Files          62      62              
  Lines       11182   11182              
  Branches     1961    1961              
=========================================
+ Hits         6618    6620       +2     
+ Misses       4011    4010       -1     
+ Partials      553     552       -1

Impacted Files	Coverage Δ
src/oic/utils/jwt.py	79.54% <100%> (ø)	⬆️
src/oic/utils/keyio.py	65.48% <0%> (+0.28%)	⬆️

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[schlenk]

LGTM.