_whichmodule changes results if multiprocessing is imported first

[hendrikmakait]

Without multiprocessing:

import cloudpickle
from numpy import exp

cloudpickle.cloudpickle._whichmodule(exp, exp.__name__) # numpy.core._multiarray_umath

Import multiprocessing last:

import cloudpickle
from numpy import exp
import multiprocessing

cloudpickle.cloudpickle._whichmodule(exp, exp.__name__) # numpy.core._multiarray_umath

Import multiprocessing first (problematic case):

import multiprocessing
import cloudpickle
from numpy import exp

cloudpickle.cloudpickle._whichmodule(exp, exp.__name__) # __mp_main__

Related pickle issue: bpo-42406