Add CI for pytest

[RemiRigal]

This PR adds a GitHub workflow for pytest testing. The deployment of the Plex server is greatly inspired from the workflow used in the repository python-plexapi.

[codecov-commenter]

Codecov Report

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The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #39   +/-   ##
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  Coverage          ?   93.11%           
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  Files             ?       18           
  Lines             ?      915           
  Branches          ?        0           
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  Hits              ?      852           
  Misses            ?       63           
  Partials          ?        0           

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claimed	93.11% <0.00%> (?)

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