Bug 1879915 - websocketprocessbridge: pin cffi to 1.14.5 for windows 7. r=jmaher, a=test-only

Pin cffi to 1.14.5 only for our Windows 7 environment, approximated as "Windows and python 3.7".

Differential Revision: https://phabricator.services.mozilla.com/D201634

testing/tools/websocketprocessbridge/websocketprocessbridge_requirements_3.txt

 # This file is the websocketprocess requirements.txt used with python 3.
 
 six
+
+# bug 1879915: cffi 1.16.0 breaks on Windows 7
+cffi==1.14.5 ; python_version < "3.8" and platform_system == "Windows"
+
 vcversioner==2.16.0.0
 twisted==21.2.0