The PR for 3.9 breaks AIX build (since https://buildbot.python.org/all/#/builders/119/builds/384) reporting the following:

Traceback (most recent call last):
  File "/data/prj/python/python3-3.9/../git/python3-3.9/setup.py", line 69, in <module>
    from distutils.command.build_ext import build_ext
  File "/data/prj/python/git/python3-3.9/Lib/distutils/command/build_ext.py", line 33, in <module>
    class build_ext(Command):
  File "/data/prj/python/git/python3-3.9/Lib/distutils/command/build_ext.py", line 63, in build_ext
    "(default: %s)" % get_platform()),
  File "/data/prj/python/git/python3-3.9/Lib/distutils/util.py", line 107, in get_platform
    return get_host_platform()
  File "/data/prj/python/git/python3-3.9/Lib/distutils/util.py", line 83, in get_host_platform
    return aix_platform()
  File "/data/prj/python/git/python3-3.9/Lib/_aix_support.py", line 83, in aix_platform
    vrmf, bd = _aix_bosmp64()
  File "/data/prj/python/git/python3-3.9/Lib/_aix_support.py", line 53, in _aix_bosmp64
    out = subprocess.check_output(["/usr/bin/lslpp", "-Lqc", "bos.mp64"])
AttributeError: module 'subprocess' has no attribute 'check_output'

Will post a PR for this.

The cause:
Briefly, due to the addition of an importable subprocess must now test specifically for check_output during the early build process.