Fix cherrypy dependency conflict

Current set of dependencies brings latest cherrypy which is
incompatible with cheroot we need. This pins cherrypy to last
version that is compatible. The current verision of pip will
still install cherrypy, but the upcoming depsolver is unable to
resolve the conflict.

We also add a fail-safe `pip check` that runs the new depsolver
and should prevent a similar regressions from appearing.

Change-Id: I2b0fa43b906d0919854d30da0ee825002444b201

requirements.txt

@@ -28,7 +28,7 @@ iso8601
 psutil
 fb-re2>=1.0.6
 paho-mqtt
-cherrypy
+cherrypy==18.3.0; # last version to support cheroot below
 ws4py
 routes
 pathspec

tools/pip.sh

@@ -39,6 +39,9 @@ then
 fi
 pip install $*
 
+# Fail-fast if pip detects conflicts
+pip check
+
 # Check if we're installing zuul. If so install the managed ansible as well.
 if echo "$*" | grep -vq requirements.txt; then
     zuul-manage-ansible -v