Allow isodatetime 3.1

[MetRonnie]

We released isodatetime 3.1.0 which was basically a bugfix release apart from dropping support for Python 3.6. The only real change in it was fixing mistake in the isodatetime command CLI help text.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• No tests needed
• No CHANGES.md entry needed
• No docs needed
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[oliver-sanders]

We should really be able to loosen this constraint further.