Do not install Celery by default, use extra [celery] instead

[Nusnus]

For testing and simple use, pip install "pytest-celery[celery]" to take the latest Celery package version.
For production environments, celery should already be installed by itself with the versioning and packages the environment uses, so pip install pytest-celery should be enough to avoid installing useless packages.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2f7517c) 27.10% compared to head (8f8ece9) 27.10%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #172   +/-   ##
=======================================
  Coverage   27.10%   27.10%           
=======================================
  Files          36       36           
  Lines        1033     1033           
  Branches      206      206           
=======================================
  Hits          280      280           
  Misses        721      721           
  Partials       32       32           

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