Distutils setup function accepts an 'options' named argument which
allows to pass options to subcommands in a dictionary (At the moment I
cannot find where this is documented).
When an empty dictionary is passed, distutils spits out a warning:

C:\sf\comtypes>py3 setup.py clean
c:\python30\lib\distutils\dist.py:259: UserWarning: Unknown distribution
option: 'options'
  warnings.warn(msg)
running clean
...

I believe the bug is in all versions of Python; since it never has been
fixed (or even found) I'm not sure if it is worth fixing.  However,
patch is attached.