ci: fix flaky tox command on Windows platform (fix #65)

[marcofavoritobi]

Fix issue #65 by parsing the output of tox looking for "success" messages (despite the internal code of tox fails).

[codecov-commenter]

Codecov Report

Merging #66 (16eeffa) into main (80a642a) will not change coverage.
The diff coverage is n/a.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main      #66   +/-   ##
=======================================
  Coverage   91.36%   91.36%           
=======================================
  Files          42       42           
  Lines        1737     1737           
=======================================
  Hits         1587     1587           
  Misses        150      150           

[marcofavoritobi]

The fix worked, e.g. see https://github.com/bancaditalia/black-it/actions/runs/6073294808/job/16474939464:

=== 83 passed, 18 skipped, 2 deselected, 191 warnings in 150.28s (0:02:30) ====
Traceback (most recent call last):
  File "C:\hostedtoolcache\windows\Python\3.9.13\x64\lib\site-packages\tox\session\cmd\run\single.py", line 105, in run_command_set
    current_outcome = tox_env.execute(
.pkg: _exit> python C:\hostedtoolcache\windows\Python\3.9.13\x64\lib\site-packages\pyproject_api\_backend.py True poetry.core.masonry.api
RuntimeError: <_overlapped.Overlapped object at 0x000001CAFC209C00> still has pending operation at deallocation, the process may crash
  py39: OK (599.33=setup[446.28]+cmd[153.05] seconds)
  congratulations :) (599.81 seconds)
Tests were successful!