After changeset e895de3e7f3cc2f7213b87621cfe9812ea4343f0, test_all fails on platforms that lack the _posixshmem extension module (Android for example):

======================================================================
FAIL: test_all (test.test___all__.AllTest) (module='multiprocessing.managers')
----------------------------------------------------------------------
Traceback (most recent call last):
File "/data/local/tmp/python/lib/python3.8/test/test___all__.py", line 34, in check_all
  exec("from %s import *" % modname, names)
AttributeError: module 'multiprocessing.managers' has no attribute 'SharedMemoryManager'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/data/local/tmp/python/lib/python3.8/test/test___all__.py", line 37, in check_all
  self.fail("__all__ failure in {}: {}: {}".format(
AssertionError: __all__ failure in multiprocessing.managers: AttributeError: module 'multiprocessing.managers' has no attribute 'SharedMemoryManager'


The following patch fixes the problem:

diff --git a/Lib/multiprocessing/managers.py b/Lib/multiprocessing/managers.py
index 7973012b98..3fdd60fff7 100644
--- a/Lib/multiprocessing/managers.py
+++ b/Lib/multiprocessing/managers.py
@@ -8,8 +8,7 @@
# Licensed to PSF under a Contributor Agreement.
#

-__all__ = [ 'BaseManager', 'SyncManager', 'BaseProxy', 'Token',
-            'SharedMemoryManager' ]
+__all__ = [ 'BaseManager', 'SyncManager', 'BaseProxy', 'Token' ]

#
# Imports
@@ -33,6 +32,7 @@ from . import get_context
try:
   from . import shared_memory
   HAS_SHMEM = True
+    __all__.append('SharedMemoryManager')
except ImportError:
   HAS_SHMEM = False