Quickfix unexpected encrypted PEM format

This quickfix changes the PEM format from PKSC8 to
PKSC5 (TraditionalOpenSSL) in pyca_crypto's variant of
`create_rsa_encrypted_pem`.
PKSC5 has the PEM headers expected by other PEM parsing functions,
e.g. `is_pem_private` and `extract_pem`.

See secure-systems-lab#54 for more details

securesystemslib/pyca_crypto_keys.py

@@ -540,7 +540,7 @@ def create_rsa_encrypted_pem(private_key, passphrase):
 
   encrypted_pem = \
     private_key.private_bytes(encoding=serialization.Encoding.PEM,
-    format=serialization.PrivateFormat.PKCS8,
+    format=serialization.PrivateFormat.TraditionalOpenSSL,
     encryption_algorithm=serialization.BestAvailableEncryption(passphrase.encode('utf-8')))
 
   return encrypted_pem.decode()