Use decorator syntax in `AesaraConfigWarning`

aesara/configparser.py

@@ -22,11 +22,10 @@ _logger = logging.getLogger("aesara.configparser")
 
 
 class AesaraConfigWarning(Warning):
+    @classmethod
     def warn(cls, message, stacklevel=0):
         warnings.warn(message, cls, stacklevel=stacklevel + 3)
 
-    warn = classmethod(warn)
-
 
 class ConfigAccessViolation(AttributeError):
     """ Raised when a config setting is accessed through the wrong config instance. """