Use get_username for compatibility with custom user models

Thanks to ProfessorLogout in
mawassk#13
for this suggestion.

Author: mpirnat

ninja_apikey/models.py

@@ -29,4 +29,4 @@ def is_valid(self):
         return self.expires_at >= timezone.now()
 
     def __str__(self):
-        return f"{self.user.username}<{self.prefix}>"
+        return f"{self.user.get_username()}<{self.prefix}>"

ninja_apikey/tests.py

@@ -53,7 +53,7 @@ def test_apikey_check():
     key.hashed_key = key_data.hashed_key
     key.save()
     assert key
-    assert user.username in str(key)
+    assert user.get_username() in str(key)
     assert not check_apikey(key_data.key)
     assert not check_apikey(key.prefix)
     assert not check_apikey(f"{key_data.prefix}.{get_random_string(10)}")