setup: bump traitlets version

[oliver-sanders]

Jupyterhub brings in traitlets v4, our usage requires v5.

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).
• Does not need tests (why?).
• No change log entry required (why? e.g. invisible to users).
• No documentation update required.
• Created an issue Bump traitlets version to >5. conda-forge/cylc-uiserver-feedstock#13

[MetRonnie]

Would suggest removing line 47 as it's become misleading

cylc-uiserver/setup.py

Lines 45 to 47 in 9685dd4

	# NB: no graphene version specified; we only make light use of it in our own
	# code, so graphene-tornado's transitive version should do.
	# Same for traitlets via jupyterhub transitive deps.

[kinow]

Leaving open so @oliver-sanders can take a look at Ronnie's comment above.