[Distutils] how do you express a platform-specific dependency?

[zooko]

Folks:

Twisted requires pywin32 if it is running on Windows.  How can the  
twisted package metadata [1], which is produced by the twisted  
setup.py [2], specify this dependency so that if you easy_install  
Twisted on Windows you'll get pywin32 as well, but if you  
easy_install Twisted on another platform you won't?

Regards,

Zooko

[1] http://pypi.python.org/pypi/Twisted
[2] http://twistedmatrix.com/trac/browser/trunk/setup.py?rev=23010#L76