Sync tox.ini with .github/workflows/python-app.yml

[living180]

Add new Python versions, update the flake8 command, and set isolated_build = True since sqlparse is using a PEP-517 pyproject.toml file (see https://tox.wiki/en/3.27.1/example/package.html for more info).

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (fc76056) 96.95% compared to head (1a4592b) 96.95%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #711   +/-   ##
=======================================
  Coverage   96.95%   96.95%           
=======================================
  Files          20       20           
  Lines        1545     1545           
=======================================
  Hits         1498     1498           
  Misses         47       47           

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[andialbrecht]

Switched to hatch today, there's no need for tox anymore.