Support Python 3.15

[leofang]

• Build & test pre-release 3.15 in the CI (and store artifacts)
• Release 3.15 wheels & conda packages when Python 3.15.0 is out
• Drop Python 3.10

Python 3.15 is schedule for October 1, 2026.

https://peps.python.org/pep-0790/

3.15.0 candidate 2: Tuesday, 2026-09-01
3.15.0 final: Thursday, 2026-10-01

[leofang]

Un-assign myself since Mike is back and we start building 3.15 wheels 🥳