Add PyLint as a development dependency

Use `pylint calsocial` to run the static analysis.

calsocial/calendar_system/gregorian.py

@@ -30,10 +30,10 @@ def to_timestamp(func):
     """
 
     @wraps(func)
-    def decorator(*args, **kwargs):
+    def _decorator(*args, **kwargs):
         return func(*args, **kwargs).timestamp()
 
-    return decorator
+    return _decorator
 
 
 class GregorianCalendar(CalendarSystem):