feat(python-multi): Add Python 3.15.0rc1

[parthea]

Towards b/525422520

[chalmerlowe]

@parthea

Do we want to install rc1?

OR

Do we want to install the latest nightly?

If we go with rc1, then as we get closer to the october release date it feels like we should regularly update the multi-image to keep pace with each new rc candidate. Feels like toil.

Since our docker images are based on ubuntu we can use the deadsnakes channel to get the latest dev version without regard for whether it is rc1 OR rc3, etc.

[parthea]

That's a fair call out regarding toil. I'd prefer rc1 because it's the version that folks are using when they download 3.15 today from https://www.python.org/downloads/. It will also provide some a higher level of stability compared to nightly builds. The toil is a one line change that we will need to do 2 more times according to https://peps.python.org/pep-0790/#release-schedule , which is acceptable IMO.

We need to change this one line 2 times over 2 months.

ARG PYTHON_VERSIONS="3.10.19 3.11.14 3.12.12 3.13.11 3.14.3 3.15.0rc1"

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