After thinking about it, is MAGIC an implementation detail? It certainly reflects changes specific to the CPython interpreter. I'm much more comfortable with leaving implementation details in Python/import.c.
On the other hand, there's already no small amount of rather CPython-specific stuff in Lib/importlib/_bootstrap.py, which belongs there. The pyc magic bytes are tightly coupled with it. Because of that, I realize, I'm still fine with the patch.
But it makes me wonder if it might be worth having a clear separation between the general and CPython-specific stuff in _bootstrap.py, for the sake of people who look at the code for the first (or tenth) time. That's the same rationale I have for advocating moving as much over from import.c as relates to the importlib implementation.