_args_from_interpreter_flags function not present causes an import error while importing multiprocessing module about python-subprocess32

[gpshead]

We dropped subprocess32 in to replace subprocess in our own internal Python build, adding the _args_from_interpreter_flags function from 2.7's subprocess to the subprocess.py file was indeed the workaround we used.

It didn't seem worth including it in subprocess32 as there is no easy way for people do naturally do this drop-in. But thinking about it, it would let us remove one patch internally in the future and make the drop-in easier for others. I'll consider it.