Don't use urllib3 1.25.4

This is broken with Gerrit due to
https://github.com/urllib3/urllib3/pull/1684

Change-Id: Ie2c817bb91463cecc64e3022e11330898b11062c

requirements.txt

@@ -29,3 +29,4 @@ cherrypy
 ws4py
 routes
 jsonpath-rw
+urllib3<=1.25.3  # https://github.com/urllib3/urllib3/pull/1684