Marc's reference to pip meant I noticed something that I had missed previously - that this issue is referring specifically to the use of setuptools/pip entry points, not to the -m switch.

Entry points shouldn't be hitting this if they're importing the module containing the entry point normally - #10845 specifically related to invoking multiprocessing from a module run directly with the -m switch, which was worked around in 3.2 and 3.3, and then finally fixed properly in 3.4 by the implementation of #19946

So if there's an incompatibility between multiprocessing and entry points, it would be preferable to fix it in pip/setuptools, as that will reach many more installations than just fixing multiprocessing to better tolerate that situation.