Fix README file extension in pyproject.toml

[lafrech]

This might be the reason PyPI doesn't show the README.

[codecov-commenter]

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Additional details and impacted files

@@          Coverage Diff           @@
##             main    #386   +/-   ##
======================================
  Coverage        ?   92.4%           
======================================
  Files           ?       5           
  Lines           ?     238           
  Branches        ?      45           
======================================
  Hits            ?     220           
  Misses          ?       7           
  Partials        ?      11           

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[corydolphin]

Thank you!