MigrationWriter prefixes builtin types with __builtins__.
|Ready for checkin
|Patch needs improvement:
When MigrationWriter finds a builtin type (i.e. not a builtin type instance) it serializes as follows:
import __builtin__ __builtin__.dict
__builtin__ isn't needed nor pretty, but worse it was renamed
builtins in PY3 so migrations written in PY2 wouldn't work in PY3 and vice versa.