remove support for cygwin

[dougxc]

The cygwin environment support in mx has always lagged behind support for other environments. I believe it was only added because that was the way someone first tried to use mx on Windows. In any case, I propose to remove support for this altogether. Otherwise, we will continue to have problems (e.g. #188 ) with people thinking it is fully supported.

Note that this does not mean you cannot use cygwin with mx. It just means you cannot use the cygwin python executable to run mx but must use a standard Windows python instead.