feature: fallback to pre-release when no stable version is found (#414)

This allows to specify version like `3.11` or `pypy3.10` in workflows before those versions are released.
This lessen the burden for users of `setup-python` by not having to modify their workflow twice: once when a pre-release is available (e.g. `3.11-dev`) and once when the first stable release is published (e.g. `3.11`)