[Tests] Fully reconfigure tox to automate everything

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