graphene/graphql-core v3 upgrade

[dwsutherland]

partially addresses cylc/cylc-uiserver#333

This is the first part described here:
cylc/cylc-uiserver#333 (comment)

The second, and arguably trickier, part will be the sibling at the UIS.

There were some breaking changes to contend with (mainly around more support for Enums):
https://github.com/graphql-python/graphene/wiki/v3-release-notes

But the most involved part was re-implementing the null stripping, but some time reading the graphql-core I found some tools to help, simplifying the code.

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).
• Tests altered and added to cover.
• Changelog entry included if this is a change that can affect users

[dwsutherland]

Another change I've found is with default_value with mutation arguments (or arguments in general)..
In v3 this value isn't used unless the argument is absent.. So setting it to None/null in the request doc is honored, whereas previously it would have used the argument default_value defined in the schema,
i.e.
This will set mode: Clean as expected:

mutation {
  stop(
      workflows: ["foo/run1"]
    ) {
    result
  }
}"

whereas this will not:

mutation {
  stop(
      workflows: ["foo/run1"],
      mode: None
    ) {
    result
  }
}"

you'll get an error, the None/null is now carried.

And we have a lot of scripts that set args to None in the options.. Slowly working through them.. They'll need to be a valid default in the options.