Bug 1605128: Make websocket_process_bridge less picky about the version of six it wants. r=ahal

Differential Revision: https://phabricator.services.mozilla.com/D60552

--HG--
extra : moz-landing-system : lando

testing/tools/websocketprocessbridge/websocketprocessbridge_requirements.txt

@@ -1,5 +1,5 @@
 # needed by txws, and we'd like pip to get it from the local server before setuptools tries pypi
-six==1.10.0
+six
 
 twisted==10.2.0