test: install pyopenssl for mtls testing (#965)

Author: arithmetic1728

synthtool/gcp/templates/python_library/noxfile.py.j2

@@ -137,6 +137,9 @@ def system(session):
     # Sanity check: Only run tests if the environment variable is set.
     if not os.environ.get("GOOGLE_APPLICATION_CREDENTIALS", ""):
         session.skip("Credentials must be set via environment variable")
+    # Install pyopenssl for mTLS testing.
+    if os.environ.get("GOOGLE_API_USE_CLIENT_CERTIFICATE", "false") == "true":
+        session.install("pyopenssl")
 
     system_test_exists = os.path.exists(system_test_path)
     system_test_folder_exists = os.path.exists(system_test_folder_path)