[3.13] gh-118658: Modify cert generation script to extract cert3.pem (GH-124598) (GH-124972)

(cherry picked from commit 480354d)

Author: felixfontein

Lib/test/certdata/cert3.pem

@@ -254,6 +254,8 @@ def print_cert(path):
         f.write(key)
         f.write(cert)
 
+    check_call(['openssl', 'x509', '-outform', 'pem', '-in', 'keycert3.pem', '-out', 'cert3.pem'])
+
     cert, key = make_cert_key('fakehostname', sign=True)
     with open('keycert4.pem', 'w') as f:
         f.write(key)