allow-prereleases: restrict to alpha, beta, rc

I maintain a C++/Python package that uses the Python's C-API. I'd like to test the package against pre-releases, but only after the ABI/API have stabilized, i.e., RCs.

Right now, setup-python has allow-prereleases which falls back to the most recent pre-release if there's no stable release yet.

I'd like to propose a more fine-grained control over which prereleases should be selected:

• (allow-prereleases: 'dev': Use dev, alpha, beta, rc prereleases)

• allow-prereleases: 'alpha': Use alpha, beta, rc prereleases (not dev, equivalent to allow-prereleases: true)

• allow-prereleases: 'beta': Use beta, rc prereleases (not dev, not alpha)

• allow-prereleases: 'rc': Use rc prereleases (not dev, not alpha, not beta)

Even simpler, one could use allow-releases:

• (allow-releases: 'dev': Use dev, alpha, beta, rc, stable releases)

• allow-releases: 'alpha': Use alpha, beta, rc, stable releases (not dev, equivalent to allow-prereleases: true)

• allow-releases: 'beta': Use beta, rc, stable releases (not dev, not alpha)

• allow-releases: 'rc': Use rc, stable releases (not dev, not alpha, not beta)

• allow-releases: 'stable' (default): Use stable releases (not dev, not alpha, not beta, not rc)

Inizia tracciando il modo in cui setup-python interpreta attualmente allow-prereleases e seleziona le versioni di Python. Confronta i livelli di pre-release proposti, determina se allow-prereleases o allow-releases è l’interfaccia prevista e verifica che ogni livello selezioni le versioni indicate senza modificare il comportamento stabile predefinito.