bpo-39481: fix test_genericalias on Android

[yan12125]

(I'm not sure if it's correct to refer bpo-39481 or not. Sorry if I did it wrong.)

Android bionic does not implement shm_open/shm_unlink [1]. As a
result _posixshmem extension does not exist and
multiprocessing.shared_memory cannot be imported.

0:01:48 load avg: 1.53 [159/421/7] test_genericalias failed
test test_genericalias crashed -- Traceback (most recent call last):
  File "/data/local/tmp/lib/python3.9/test/libregrtest/runtest.py", line 270, in _runtest_inner
    refleak = _runtest_inner2(ns, test_name)
  File "/data/local/tmp/lib/python3.9/test/libregrtest/runtest.py", line 221, in _runtest_inner2
    the_module = importlib.import_module(abstest)
  File "/data/local/tmp/lib/python3.9/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
  File "<frozen importlib._bootstrap>", line 986, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 680, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 790, in exec_module
  File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
  File "/data/local/tmp/lib/python3.9/test/test_genericalias.py", line 22, in <module>
    from multiprocessing.shared_memory import ShareableList
  File "/data/local/tmp/lib/python3.9/multiprocessing/shared_memory.py", line 23, in <module>
    import _posixshmem
ModuleNotFoundError: No module named '_posixshmem'

[1] https://android.googlesource.com/platform/bionic/+/master/docs/status.md

https://bugs.python.org/issue39481

Automerge-Triggered-By: @gvanrossum

[gvanrossum]

Thanks, good catch.

[yan12125]

Thanks!