What makes it worse, is that while superficially, Unicode versions follow the same X.Y.Z format as Python versions, the stability promises are completely different. For example, it appears that the general category for the ZERO WIDTH SPACE was changed in Unicode 4.0.1. I don't think a change affecting str.split(), int(), float() and probably numerous other library functions would be acceptable in a Python micro release.
Well, we managed to completely break Unicode normalization between 2.6.5 and 2.6.6, due to a bug.
You can see the Unicode Consortium's stability policy at
In a sense, this is stronger than Python's backwards compatibility promises (which allow for certain incompatible changes to occur over time, whereas Unicode makes promises about all future versions).