osx: wire up genosxversion script
The only version strings that are changed are the ones baked into the
.pkg - hg's self-reported version string doesn't change, so users will
still see our mostly-pip-compatible version strings.
For reference, the part of our versioning setup that's not PEP440
compatible is the RC releases - those should be .rc0 insted of
-rc. It's too late to change that for the 4.3 cycle, so I'll worry
about fixing that during the 4.4 cycle.