Remove pytest-randomly from the dev dependencies

- problem: because of the randomization of test cases, every once in a
  while, the schema holding the test db already exists and cannot be
  created a second time (background: we run all tests against a db
  created with `metadate.create_all()` by SQLAlchemy at once and
  a db created by running all incremental Alchemy migration scripts)
- quick fix: possible, we could find a way to run all tests against
  one of the two test db's in random order and then against the other
  => in the interest of time, we simply do not randomize the test cases