ci: Mark Python 3.10 as experimental

[The-Compiler]

It currently breaks all builds and even after #8540 it looks like tests
still fail because of benjaminp/six#341

As a hotfix, mark it as experimental until everything is fixed, so that
our CI isn't all red.

[The-Compiler]

Hm, looks like CI still is marked as failed. Not sure what I'm missing?

[nicoddemus]

Thanks Florian!

But I'm not sure we need this: I did research this a bit previously (can't look for the links now), but seems the official GH Actions stance on "marking a build as allowed to fail" is to make the build not required in the repository settings, which we already do:

The effect is that even though 3.10 has failed here, the "merge" button is still enabled (in the screenshot above if ubuntu-py36 failed, then the merge button wouldn't be enabled).

[The-Compiler]

GitHub documents it as:

You can allow specific jobs in a job matrix to fail without failing the workflow run.

And indeed looking at the icon at the top left, the workflow run passed - but it doesn't seem to affect the PRs being marked as failure. So yeah, let's close this, as it doesn't really help much after all.