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On macOS, if running micropython from subprocess.check_output, then a
stdout.flush() raises error 45.

Here's a test case.  This will run fine on linux, but crashes on macOS with
error 45.

    import sys
    import subprocess
    import tempfile
    with tempfile.NamedTemporaryFile('w') as fp:
        fp.write('''
    import sys
    sys.stdout.write('hello world')
    sys.stdout.flush()
    print('')
    ''')
        fp.flush()
        print('py3')
        o = subprocess.check_output(f'python3 {fp.name}'.split())
        print(o)
        print('upy')
        o = subprocess.check_output(f'micropython {fp.name}'.split())
        print(o)

On macOS:

    py3
    b'hello world\n'
    upy
    Traceback (most recent call last):
      File "...", line 4, in <module>
    OSError: 45

On unix:

    py3
    b'hello world\n'
    upy
    b'hello world\n'
Signed-off-by: stephanelsmith <stephane.smith@titansensor.com>