Test Python 3.12-dev

[hugovk]

For #6941.

Fixes #6942.

Changes proposed in this pull request:

• Python 3.12 is in alpha and I expect we'll release wheels during the beta
• Let's start testing now
• Not included Windows yet, see Python 3.12: Fix building Pillow on Windows #6942

[radarhere]

...I meant to add that commit to my own repository, but accidentally pushed it here instead.

It re-adds Focal to the CI targets table, because we are still testing that through our Docker builds, and removes 'x86-64' from the list of Python 3.10 Jammy builds, as it it already listed in the previous row.

[hugovk]

That's fine, thank you for the update!