I'd like to reopen this, as it doesn't seem to be a duplicate of #8084. Specifically, test_getsitepackages in test_sitepackages appears to be wrong, since it has a correct test for platform builds later in the method, but the failure occurs earlier because the path "os.sep == '/'" is taken on OS X. The test should be restructured as e.g.

    if sys.platform in ('os2emx', 'riscos'):
        # stuff for OS/2, RISCOS
    elif (sys.platform == "darwin" and
            sysconfig.get_config_var("PYTHONFRAMEWORK")):
        # OS X platform builds
    elif os.sep == '/':
        # OS X non-platform builds, Linux, FreeBSD etc.
    else:
        # other platforms

That the test itself is broken is not the thrust of #8084.