We change the command-line option to a version recommended in
the pytest docs:
https://docs.pytest.org/en/latest/example/markers.html#custom-marker-and-command-line-option-to-control-test-runs

- A global `suite(name)` mark is registered, that takes a
  name and marks a test as belonging to a specific suite.
- These special suites are opt-in, i.e., not run by default.
- To enable a suite, one adds `-S name` to the pytest
  command-line options.
- Since function-scoped fixtures are handled stricter now,
  the workaround in tensors_test.py didn't work any longer
  (see pytest-dev/pytest#6497).
- We thus set the minimum pytest version to 5.4.0, where the
  fix for the issue is implemented.

Closes: odlgroup#1514