This reverts commit ce766c73.

It calls black and isort on the source tree.

The following separate testenvs are defined (with their commands to run):

  tox              Run test suite
  tox -e lint      Run flake8 tests (including mypy and bandit)
  tox -e security  Run safety to check for vulnerable dependencies
  tox -e docs      Run sphinx to build documentation
  tox -e build     Run poetry to build sdist and bdist_wheel

Very bad design in Django code...

* Use implicit skip of SeleniumTestCase if no browsers are selected
* Allow selecting browsers from TEST_SELENIUM_BROWSERS environment variable
* Allow defining listen address and HTTP host separately