Fix Pylint issues

[driverpt]

ERRORS:
<class 'rest_framework_api_key.admin.APIKeyAdmin'>: (admin.E108) The value of 'list_display[4]' refers to 'has_expired_func', which is not a callable, an attribute of 'APIKeyAdmin', or an attribute or method on 'rest_framework_api_key.APIKey'.

[florimondmanca]

Hi,

Thanks for taking the time to open this PR. This field is actually required and dynamically added by the @.djangoproperty decorator. It is a work around the fact that the admin requires a reference to the property’s getter function to properly display Booleans as red/green icons.

To fix the Pylint issue, we can add a type annotation on the model instead. Can you try this out?

[florimondmanca]

@driverpt I didn't realize this was a system check error. This has been reported in #38 and a fix should be released soon. In the meantime we can see #38 for a hot fix. I'll close this since removing the field is a hot fix too, but it won't actually solve the problem. Stay tuned for a patch. Thanks!