chore: start 3.2.0a0 development and block final-version nightlies (#6191)

Master was left at 3.1.0 final, so nightly wheels had the same version as
the PyPI release. Bump to 3.2.0a0 and make the nightly upload fail when a
wheel does not have a prerelease version.

Fixes #6182.

Assisted-by: ClaudeCode:claude-opus-5-5