Trac #21598: psutil: platform cygwin not supported

Installing psutil on cygwin simply doesn't work.  It actually refuses to
install on cygwin.  See attached log.

The relevant upstream issue for psutil is here:
giampaolo/psutil#82

My in-progress pull request is here:
giampaolo/psutil#998

The attached patch is based on the current version of my pull request,
where all (relevant) tests pass on Cygwin.  The code itself is a bit of
a mess and needs refactoring, so it will take a little more time before
I can get this accepted upstream.  But in the meantime I will maintain
this patch for Sage.

URL: https://trac.sagemath.org/21598
Reported by: embray
Ticket author(s): Erik Bray
Reviewer(s): Jeroen Demeyer

build/pkgs/psutil/package-version.txt

@@ -1 +1 @@
-5.2.0
+5.2.0.p0