Python 3.10 support

[MetRonnie]

Seeing as there wasn't already an issue open for this...

Two blockers:

• [ ] graphene-tornado 2.6 has a dependency on werkzeug 0.12 that doesn't support py3.10

  File "~/.conda/envs/cylc8.1/lib/python3.10/site-packages/werkzeug/datastructures.py", line 16, in <module>
    from collections import Container, Iterable, MutableSet
  ImportError: cannot import name 'Container' from 'collections'
  

  (Devs can patch this file in site-packages by changing it to collections.abc)

• [x] #390

Pull requests welcome!