There was some concern with PyImport_GetMagicTag() extracting its value from sys.implementation.cache_tag.  One solution is to have the two use a common underlying value.  However, that's basically what was already in import.c and I'd rather minimize that file at this point.

An alternative is to have the underlying value defined in a more amenable spot, but I'm not convinced that buys a whole lot over simply exposing the underlying value in PyImport_GetMagicTag().  If we could deprecate that function...

Another possibility is to move the underlying value to configure.  The down-side is that it's platform-specific.  I've attached a patch that does this.  While it may do too much as-is, it demonstrates my point.