This line is what does the trick:

Basically, on Windows, you'll automatically get the `twisted-iocpsupport` module as a hard dependency.  On other platforms, you won't.  Unless you're using a truly ancient `pip` (which is less likely on Windows, where people tend to install things themselves rather than getting them from a calcified platfom) this will just work ✨ magically ✨.

(Personally I think that in a future change, we should seriously consider moving this to an extra, since for some Windows users the simplicity of a pure-python install outweighs the performance improvements of a binary dependency.  And it should be possible to install Twisted without this accelerator.  But that is a very minor point compared to the benefits of not having to drag platform-variant wheels around for all other platforms!)

-g