Test refactoring

[elacuesta]

1. Updated tests to use the most recent versions of pytest plugins (previously not working with pytest-asyncio>=0.11)
2. Split asyncio/twisted tests, they don't play well together
3. Bump minimum Python version to 3.8, to use unittest.IsolatedAsyncioTestCase (py37 is EOL anyway)
4. Pinned playwright==1.36.0 in tests, playwright==1.37.0 fails for some reason

[codecov]

Codecov Report

Merging #223 (f280f4f) into main (1c3f425) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #223   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines          388       388           
=========================================
  Hits           388       388           

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