bpo-30028: make test.support.temp_cwd() fork-safe (GH-1066)

Make test.support.temp_cwd() fork-safe. The context manager test.support.temp_cwd() no longer removes the temporary directory when executing in a process other than the parent it entered from.
If a forked child exits the context manager it won't do the cleanup.
diff --git a/Misc/ACKS b/Misc/ACKS
index 2eddc56..dee022f 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -857,6 +857,7 @@
 Hannu Krosing
 Andrej Krpic
 Ivan Krstić
+Anselm Kruis
 Steven Kryskalla
 Andrew Kuchling
 Dave Kuhlman