ci: unify Python version to 3.10 across single-version CI lanes (#3421)

ci: unify Python version to 3.10 across CI lanes

Several CI jobs were hardcoded to Python 3.12 while the default matrix
and the project's minimum supported version is 3.10. This caused PR
runs (e.g. #3414) to mix 3.10 and 3.12, hiding 3.10-specific issues
behind 3.12-only checks and adding noise to the CI summary.

Switch the single-version CI lanes (lint, tach, pr-fast-checks,
windows/macOS smoke tests, build-package, test-package) to 3.10 so
they match the default matrix. Release/publishing flows (cd.yml,
pypi.yml) keep their existing 3.12 pin since they target the
release toolchain rather than the CI matrix.

Signed-off-by: Georg Heiler <georg.kf.heiler@gmail.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>