py install 3-dev doesn't install prerelease #428

It should be installing 3.16.0a1, but it's still trying to get 3.15.0. py install 3.16-dev works.

Somewhere in the install/match logic we are checking whether the requested tag is prerelease and using that to allow selecting a prerelease version on the first pass. Clearly that isn't working properly and needs to be fixed.