Add parallel update path for prereleases. #206
                
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This allows us to make prerelease releases that are available from python.org and will automatically update through subsequent prereleases until the next real release. Prereleases are not added to the Store.
It's currently not clear whether an automatic update from a prerelease to a real release will later automatically update back to a prerelease - that's entirely in the hands of the OS, which decides whether it will replace auto-update settings for a package. Either way, since our intended release model is for updates as needed and only infrequent feature development, there's no reason to think we'll spend much time in prereleases.