[2.7] Clear potential ref cycle between Process and Process target (GH-2470) (#2473)

* 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 79d37ae979a65ada0b2ac820279ccc3b1cd41ba6)
2 files changed