Python finally offloads some batteries

https://bugs.python.org/issue27777 was an absolute blocker in my application causing extremely confusing failures (and that only due to quite pedantic tests - we might easily have missed it until it hit production), and caused me weeks of work trying to work around it before I eventually concluded that cgi.FieldStorage was engaged in playing core-wars with other bits of itself and there was no realistic prospect of it ever being fixed, so switched to something different instead. See https://github.com/zopefoundation/zope.publisher/issues/39.