python3: remove _subprocess (#4022)

This doesn't seem to exist in Python 3.5, which is the earliest Python3
we support, and this folder is in stdlib/3

Co-authored-by: hauntsaninja <>

tests/stubtest_whitelists/py3_common.txt

@@ -3,7 +3,6 @@ _dummy_threading
 _importlib_modulespec
 _operator.methodcaller
 _posixsubprocess.cloexec_pipe
-_subprocess
 _types
 _weakref.CallableProxyType.__getattr__
 _weakref.ProxyType.__getattr__