Change DSA test key format + adopt test code

A recent pyca/cryptography update dropped support for DSA ssh
public keys with bit size other than 1024, which broke our tests.
See pyca/cryptography#5373

This commit changes the test key format to X.509 SubjectPublicKeyInfo PEM
and updates the corresponding test code.

```
ssh-keygen -f C242A830DAAF1C2BEF604A9EF033A3A3E267B3B1.ssh \
   -e -m pkcs8 > C242A830DAAF1C2BEF604A9EF033A3A3E267B3B1.pem
```

Note: ssh-keygen mistakingly calls the format pkcs8 although it
is X.509 SubjectPublicKeyInfo PEM.

tests/gpg_keyrings/dsa/C242A830DAAF1C2BEF604A9EF033A3A3E267B3B1.pem

@@ -0,0 +1,20 @@
+-----BEGIN PUBLIC KEY-----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+-----END PUBLIC KEY-----

tests/test_gpg.py

@@ -655,31 +655,31 @@ def tearDownClass(self):
  def test_export_pubkey(self):
  """ export a public key and make sure the parameters are the right ones:
 
- since there's very little we can do to check rsa key parameters are right
- we pre-exported the public key to an ssh key, which we can load with
- cryptography for the sake of comparison """
+ since there's very little we can do to check key parameters are right
+ we pre-exported the public key to an x.509 SubjectPublicKeyInfo key,
+ which we can load with cryptography for the sake of comparison """
 
  # export our gpg key, using our functions
  key_data = export_pubkey(self.default_keyid, homedir=self.gnupg_home)
  our_exported_key = dsa_create_pubkey(key_data)
 
- # load the equivalent ssh key, and make sure that we get the same RSA key
- # parameters
- ssh_key_basename = "{}.ssh".format(self.default_keyid)
- ssh_key_path = os.path.join(self.gnupg_home, ssh_key_basename)
- with open(ssh_key_path, "rb") as fp:
+ # load same key, pre-exported with 3rd-party tooling
+ pem_key_basename = "{}.pem".format(self.default_keyid)
+ pem_key_path = os.path.join(self.gnupg_home, pem_key_basename)
+ with open(pem_key_path, "rb") as fp:
  keydata = fp.read()
 
- ssh_key = serialization.load_ssh_public_key(keydata,
+ pem_key = serialization.load_pem_public_key(keydata,
  backends.default_backend())
 
- self.assertEqual(ssh_key.public_numbers().y,
+ # make sure keys match
+ self.assertEqual(pem_key.public_numbers().y,
  our_exported_key.public_numbers().y)
- self.assertEqual(ssh_key.public_numbers().parameter_numbers.g,
+ self.assertEqual(pem_key.public_numbers().parameter_numbers.g,
  our_exported_key.public_numbers().parameter_numbers.g)
- self.assertEqual(ssh_key.public_numbers().parameter_numbers.q,
+ self.assertEqual(pem_key.public_numbers().parameter_numbers.q,
  our_exported_key.public_numbers().parameter_numbers.q)
- self.assertEqual(ssh_key.public_numbers().parameter_numbers.p,
+ self.assertEqual(pem_key.public_numbers().parameter_numbers.p,
  our_exported_key.public_numbers().parameter_numbers.p)
 
  def test_gpg_sign_and_verify_object_with_default_key(self):