Following PyCQA/astroid@206d8a2 we sarted getting a whole bunch of
errors like
E:266,44: Value 'headers' doesn't support membership test
and
E:267,25: Value 'headers' is unsubscriptable
Digging around a bit, apparently astroid thinks the headers returned
from call_app will always be None -- I guess it doesn't like our use of
a list to work around py2's lack of `nonlocal`.
By using a proper object to encapsulate state, we can shut up those
"error"s.
Also, pin upper-constraints to pike for keystone-related jobs.
Change-Id: I5ff21260872f4089b030cd94e494dc346ae74b8e
b3efa1465c