[3.6] Clear potential ref cycle between Process and Process target (GH-2470) (#2471)

* Clear potential ref cycle between Process and Process target

Besides Process.join() not being called, this was an indirect cause of bpo-30775.
The threading module already does this.

* Add issue reference.
(cherry picked from commit 79d37ae979)

Lib/multiprocessing/process.py

@@ -104,6 +104,9 @@ class BaseProcess(object):
         _cleanup()
         self._popen = self._Popen(self)
         self._sentinel = self._popen.sentinel
+        # Avoid a refcycle if the target function holds an indirect
+        # reference to the process object (see bpo-30775)
+        del self._target, self._args, self._kwargs
         _children.add(self)
 
     def terminate(self):