fix problem with python3.6

[chilic]

Related issues:
#223
#207

[codecov-io]

Codecov Report

Merging #224 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #224   +/-   ##
=======================================
  Coverage   96.05%   96.05%           
=======================================
  Files           5        5           
  Lines         482      482           
=======================================
  Hits          463      463           
  Misses         19       19

Impacted Files	Coverage Δ
oauth2/__init__.py	100% <100%> (ø)	⬆️

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