Update optimade dependency to support Python 3.11

[danielhollas]

Just reporting my observation when trying to build AiiDAlab docker image with Python 3.11.
Unfortunately optimade==0.18 that is pinned by this package had a Python version constraint <3.11.
It looks like updating it to 0.21.0 would solve this (but I have no idea if that would require any code changes in optimade-client).

Note we're currently in no rush to update to 3.11 just reporting my findings for future reference.

Note that the optimade package removed the Python upper constraint entirely, so future updates to 3.12 and beyond should hopefully be less painful.