Python 3.9 support invalid

[CasperWA]

In #660 we have seen that using pydantic < 1.7 fails the Python 3.9 CI job - which it should, because Python 3.9 support was not added until pydantic v1.7 (see changelog).

So official Python 3.9 support should be removed until we can support pydantic >= 1.7, which there is an open PR for: #715.

[ml-evs]

Why hasn't this been failing for other PRs? I thought I'd just broken something by adding config values

[CasperWA]

No idea. But I don't want to waste time figuring it out. I'd rather just acknowledge that v1.6 of pydantic cannot be expected to support Python 3.9 and try instead to update our pydantic dependency.

[CasperWA]

An idea: You can try to install the #660 branch in a Python 3.9 environment, updating to the latest pydantic version (1.8) and run pytest?

[ml-evs]

An idea: You can try to install the #660 branch in a Python 3.9 environment, updating to the latest pydantic version (1.8) and run pytest?

I can confirm that everything passes

[CasperWA]

Closed by updating to pydantic~=1.8 in #731.