append .sexp extension to generated 'gpg-key' file

.gitignore

@@ -1,2 +1,2 @@
 /cert-request.pem
-/gpg-key
+/gpg-key.sexp

create-gpg-shadow-key-from-x509-cert-req.py

@@ -39,5 +39,5 @@ def convert(data):
 
 key = convert(key_data)
 
-with open('gpg-key', 'wb') as f:
+with open('gpg-key.sexp', 'wb') as f:
     f.write(key)