Ok, I've made some progress on this. The attached patch now generates
MSIs which are version agnostic and look up the appropriate Python
version in the registry. Some things still remaining to do:

* The ProductName needs to be modified at runtime to prefix the "Python
X.Y" for Add/Remove Programs (ARP). I have a custom action making the
appropriate modifications, but this doesn't seem to affect the name in
ARP. Not entirely sure what the problem is, but it seems I may only be
setting the property on the client, and not on the server. I'll continue
looking into this.

* I need to thoroughly test that it does the right thing when a
particular version of Python is required. The code should basically be
there now to do that, but I haven't tested it at all. Probably I need to
play around both with target_version and actually building an extension
module.

Speaking of tests, I have no idea how I would write a unittest for any
of these things. To check some of them, it seems like you'd actually
have to install the .msi onto a Windows machine. So at the moment, all
my tests are by hand. Any better suggestions for testing these kinds of
things would be greatly appreciated.