add py3.12 CI and update pyproject.toml

[dcherian]

What is your issue?

We haven't done this yet!

xarray/pyproject.toml

Lines 11 to 15 in d87ba61

	"Programming Language :: Python :: 3",
	"Programming Language :: Python :: 3.9",
	"Programming Language :: Python :: 3.10",
	"Programming Language :: Python :: 3.11",
	"Topic :: Scientific/Engineering",

[jhamman]

I think @keewis tried 3.12 back in Nov but it wasn't ready. #8416. Would be great to revisit.

[keewis]

👍

Typically it's numba that takes a while, so we'd need to remove it (and sparse / numbagg) from the environment. For 3.11 I created a separate environment file (two files, all and windows), we could do the same for 3.12