Bump bundled Python to v3.11

[jaimergp]

Comes from napari/napari-plugin-manager#108 (comment)

While we are at this, I'd like to formalize our policy for the bundled Python version, because we keep discussing this and the outcome is slightly different every time. I thought we wanted to use the oldest supported Python version for each release. As of today, 3.8 is still supported for one more month, 3.9 for one more year (more details). If that's "too old", what's the policy?

• SPEC0: we would take 3.11, as 3.10 is now recommended to drop (as of today lol)
• NEP29: that'd give us 3.10 now.
• conda-forge pinnings, which don't follow a particular schedule, but consider it more conservative than NEP 29. At 3.9 now.
• Python's EOL schedule: still at 3.8 for one more month, 3.9 for one more year.

This PR implements NEP29, but happy to adjust.

[jni]

@jaimergp if we were going with "oldest", at any rate it would be "oldest according to SPEC-0", which just became 3.11.

[jni]

(As far as I'm concerned SPEC-0 supersedes NEP-29 for packages other than NumPy.)

[jaimergp]

Got it, will update. I'm curious, though, why is napari@main still on 3.9?

[jni]

why is napari@main still on 3.9?

Inertia, and also this old thread from Talley has made me at least slightly less gung-ho to rip stuff out until I see a need to do so. (e.g. 3.9 failing for some annoying reason would make me pull off the bandaid.)

But anyway I think we were meant to remove 3.9 after 0.5.0 was released, maybe even 0.4.19, it just hasn't been a priority.