Drop Python 3.9

[crusaderky]

The latest version of several of the libraries that integrate with array-api-compat doesn't support Python 3.9 anymore. This produces the headache of having a de-facto, undocumented minimum supported version of those libraries, which is going to become more and more obsolete as time goes by, unwittingly enforced by the Python 3.9 CI jobs.

I think array-api-compat should follow the rest of the industry and formally drop python 3.9 support.

[ev-br]

scikit-learn still supports python 3.9, IIUC

[NeilGirdhar]

Does this project not follow SPEC 0?

[ev-br]

Not automatically, no.
Downstream consumers may or may not follow SPEC 0 for various reasons, and we should not exert an undue pressure on them.

We discussed this in the last consortium meeting. The outcome is that we'll likely be dropping python 3.9 and numpy 1.21 after the next release.