pythongh-118658: Modify cert generation script to extract cert3.pem (pythonGH-124598)

Author: felixfontein

Lib/test/certdata/cert3.pem

@@ -266,6 +266,8 @@ def write_cert_reference(path):
         f.write(key)
         f.write(cert)
 
+    check_call(['openssl', 'x509', '-outform', 'pem', '-in', 'keycert3.pem', '-out', 'cert3.pem'])
+
     cert, key = make_cert_key(cmdlineargs, 'fakehostname', sign=True)
     with open('keycert4.pem', 'w') as f:
         f.write(key)