This check follows CPython's behaviour, because 'import *' always populates
the globals with the imported names, not locals.
Since it's safe to do this (doesn't lead to a crash or undefined behaviour)
the check is only enabled for MICROPY_CPYTHON_COMPAT.
Fixes issue #5121.
The way it was written previously the variable x was not an implicit
nonlocal, it was just a normal local (but the compiler has a bug which
incorrectly makes it a nonlocal).
Petr Viktorin