Improve check for deprecated flags

Checking with getattr(object, name, [default]) provides
false positivies for flags that have their value set to None.
Checking with try: getattr(object, name) always produces
correct check values if a flag is defined in project settings.
Default values for flags are defined in the Axes default settings
so this should not alter any existing behaviour.

axes/checks.py

@@ -99,11 +99,14 @@ def axes_deprecation_check(app_configs, **kwargs):  # pylint: disable=unused-arg
     ]
 
     for deprecated_setting in deprecated_settings:
-        if getattr(settings, deprecated_setting, None) is not None:
+        try:
+            getattr(settings, deprecated_setting)
             warnings.append(Warning(
                 msg=Messages.SETTING_DEPRECATED.format(deprecated_setting=deprecated_setting),
                 hint=None,
                 id=Codes.SETTING_DEPRECATED,
             ))
+        except AttributeError:
+            pass
 
     return warnings