See https://docs.python.org/3/using/cmdline.html#envvar-PYTHONHASHSEED

Setting a value will set the environment variable for all processes
and for the test runner.

Default behavior is:
  - for zope, do not set the variable, so default python behavior will
  be used ( equivalent to setting `0` on python2 and `random` on python3)
  - for test runner, generate a random value for each execution and
  print it, to make it easy to re-run a failing test with the same seed.