Doesn't appear in gVim on Windows, need to set the panose attribute first:

  from fontTools import ttLib
  from path import Path

  output_dir = Path('mono')
  for file in Path('.').files('*.otf'):
      print(file)
      font = ttLib.TTFont(file)
      font['post'].isFixedPitch = 1
      font['OS/2'].panose.bProportion = 9
      font.save(output_dir / file.name)
      font.close()