Added Python 3.11 classifier

[pfmoore]

No description provided.

[pfmoore]

Unless anyone objects, I'll commit this later tonight.

[sbidoul]

Not objecting, but should we add 3.11 to our CI at the same time?

[pfmoore]

Maybe. I don't know how Github Actions tracks Python pre-releases - could we just add "3.11" to the list of versions, or do we have to include something to say we're happy with a beta/rc version? Looking back through the git logs, it looks like (c9b234c) we do need to do something special for pre-releases.

If someone wants to do a "proper" job with this, adding 3.11 pre-releases to CI, updating the docs to state that we support 3.11, adding the classifier in line with that, then I'm fine with that. I thought @pradyunsg's comment here was intended to mean that we just add the classifier in the first instance. But if not, then let's drop this and wait until someone gets round to doing the whole exercise.

[sbidoul]

Trying that python 3.1 CI in #11285