fix python3.7 compatibility

[jettify]

No description provided.

[codecov]

Codecov Report

Merging #97 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master      #97   +/-   ##
=======================================
  Coverage   98.63%   98.63%           
=======================================
  Files           1        1           
  Lines         293      293           
  Branches       41       41           
=======================================
  Hits          289      289           
  Misses          2        2           
  Partials        2        2

Impacted Files	Coverage Δ
janus/__init__.py	98.63% <0%> (ø)	⬆️

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