Fix pandas warning

[raphaelvallat]

This PR fixes an annoying pandas 1.5+ warning, reported in #322 and #311.

I have also added Python 3.10 to unit tests

[codecov]

Codecov Report

Base: 98.59% // Head: 98.59% // No change to project coverage 👍

Coverage data is based on head (16b3432) compared to base (7e93cf9).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #323   +/-   ##
=======================================
  Coverage   98.59%   98.59%           
=======================================
  Files          19       19           
  Lines        3343     3343           
  Branches      540      540           
=======================================
  Hits         3296     3296           
  Misses         25       25           
  Partials       22       22           

Impacted Files	Coverage Δ
pingouin/plotting.py	95.72% <ø> (ø)
pingouin/distribution.py	99.41% <100.00%> (ø)
pingouin/parametric.py	99.56% <100.00%> (ø)

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