chore(tox): drop --no-build-isolation from Cython tests

[vytas7]

Closes #2184

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (89bfb57) to head (81c51fd).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2229   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           63        63           
  Lines         6931      6931           
  Branches      1258      1258           
=========================================
  Hits          6931      6931           

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[CaselIT]

I don't remember if we mentioned something also in the docs / readme regarding the build isolation

[vytas7]

@CaselIT you'd be right, installation docs are very outdated... I'll create a separate issue tracking this.

Edit: filed as #2232.