Test with Python 3.10

[kinow]

This is a small change with no associated Issue.

Was adding Python 3.10 to another Py project, decided to add it to Cylc UIS too since UIS is simpler than Flow. I think pyuv needs updating before UIS works with 3.10, but at least this will remind us to update that eventually. Using experimental ¹ so the jobs don't fail.

Can be merged as-is, or left here until pyuv is fixed. No worries if superseded by another PR too 🤷 👍

Requirements 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).
• Already covered by existing tests.
• No change log entry required (why? e.g. invisible to users).
• No documentation update required.
• No dependency changes.

Footnotes

1. https://docs.github.com/en/actions/learn-github-actions/workflow-syntax-for-github-actions#example-preventing-a-specific-failing-matrix-job-from-failing-a-workflow-run ↩

[codecov-commenter]

Codecov Report

Merging #260 (19c01e5) into master (b4f6711) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #260   +/-   ##
=======================================
  Coverage   75.00%   75.00%           
=======================================
  Files          11       11           
  Lines         748      748           
  Branches      127      127           
=======================================
  Hits          561      561           
  Misses        161      161           
  Partials       26       26           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b4f6711...19c01e5. Read the comment docs.

[MetRonnie]

Blocked by werkzeug 0.12 which is a transitive dependency via graphene-tornado 2.6. See #388

[oliver-sanders]

Blocked by #333