Retire support for Python=3.9

[valeriupredoi]

Python 3.9 accepts only security updates, as this nice plot tells us - in fairness, there's a lengthy time until it reaches EOL, but I reckon, with merging #2227 by which we get support for 3.12 in, we may think of retiring support for 3.9 for the next release? Opinions welcome 🍺

PS I know some users depend on Python 3.10 environments eg @rebeccaherman1 - so we should think twice, in the not so distant future, about 3.10

[bouweandela]

As you know, we follow NEP-29 and will support Python 3.9 until April 5, 2024.

[valeriupredoi]

which date falls between (our) releases, that's why this issue - do we support it past 2.11 and retire it with 2.12?

[bouweandela]

2.12

[valeriupredoi]

tres bien, mon capitain 🍺