Fix pickle bug with pandas 1.3.0

[JarnoRFB]

Closes #37

[codecov]

Codecov Report

Merging #38 (dfb3c28) into master (ed297b4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #38   +/-   ##
=======================================
  Coverage   83.07%   83.07%           
=======================================
  Files          30       30           
  Lines         638      638           
=======================================
  Hits          530      530           
  Misses        108      108           

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