pinning to versions of omero-py that have been tested

[erickmartins]

Description

This PR changes our approach to defining omero-py versions to be much more permissive. I have tested current ezomero version all the way back to omero-py=5.9.0 and tests pass, but I'm being conservative and pinning to >=5.13.0, <5.22.0.

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.

[erickmartins]

This fixes #103.