Bump Isodatetime and add simple `cylc validate` test

[MetRonnie]

We forgot to change isodatetime to 1!3.0.*

Checklist

• [x] Used a personal fork of the feedstock to propose changes
• [x] Bumped the build number (if the version is unchanged)
• [x] Re-rendered with the latest conda-smithy (Use the phrase code>@<space/conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• [x] Ensured the license file is being packaged.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[MetRonnie]

@conda-forge-admin, please rerender

[github-actions[bot]]

Hi! This is the friendly automated conda-forge-webservice.

I tried to rerender for you, but it looks like there was nothing to do.

This message was generated by GitHub actions workflow run https://github.com/conda-forge/cylc-flow-feedstock/actions/runs/2376909164.

[hjoliver]

(Oops I already merged a PR for the isodatetime change! Do you want to rebase this, to keep the new test?)

[MetRonnie]

I'll keep this branch and add it (along with my jinja2 v2->3 branch) to the rc4 release

[MetRonnie]

Branch here for when the time comes: https://github.com/MetRonnie/cylc-flow-feedstock/tree/jinja2