- attempt to fix failure on macOS by pre-installing cffi

.github/workflows/tests.yml

@@ -297,6 +297,7 @@ jobs:
           pip install -U wheel setuptools
           pip install -U coverage
           pip install -U 'faulthandler; python_version == "2.7" and platform_python_implementation == "CPython"'
+          pip install -U 'cffi; platform_python_implementation == "CPython"'
           # Unzip into src/ so that testrunner can find the .so files
           # when we ask it to load tests from that directory. This
           # might also save some build time?