The migrations loader prevents the use of PEP-420 namespace packages
for holding apps' migrations modules. Previously the loader tested for
this only by checking that app.migrations.__file__ is present. This
prevented migrations' being found in frozen Python environments that
don't set __file__ on any modules. Now the loader *additionally* checks
whether app.migrations.__path__ is a list because namespace packages
use a different type for __path__. Namespace packages continue to be
forbidden, and, in fact, users of normal Python environments should
experience no change whatsoever.
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