Bugfix/make python39 compatible

[b8raoult]

No description provided.

[FussyDuck]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.

Project coverage is 88.23%. Comparing base (a929a5b) to head (034aca8).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
tests/test_dates.py	0.00%	4 Missing ⚠️
tests/test_frequency.py	0.00%	4 Missing ⚠️
tests/test_utils.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop       #8      +/-   ##
===========================================
- Coverage    91.83%   88.23%   -3.61%     
===========================================
  Files            3        3              
  Lines           98      102       +4     
===========================================
  Hits            90       90              
- Misses           8       12       +4     

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