If you want to use --fail-env-changed with PR 2841, for example to use test.bisect, you may want this extra patch:

diff --git a/Lib/test/_test_multiprocessing.py b/Lib/test/_test_multiprocessing.py
index f2342c3..5f9e5c3 100644
--- a/Lib/test/_test_multiprocessing.py
+++ b/Lib/test/_test_multiprocessing.py
@@ -4352,6 +4352,7 @@ class ManagerMixin(object):
                       "children after %s seconds"
                       % (multiprocessing.active_children(), dt),
                       file=sys.stderr)
+                test.support.environment_altered = True
                 break
 
         gc.collect()                       # do garbage collection
@@ -4359,6 +4360,7 @@ class ManagerMixin(object):
             # This is not really an error since some tests do not
             # ensure that all processes which hold a reference to a
             # managed object have been joined.
+            test.support.environment_altered = True
             print('Warning -- Shared objects which still exist at manager '
                   'shutdown:')
             print(cls.manager._debug_info())