Replace deprecated logger.warn with logger.warning

Author: Pacu2

django_measurement/models.py

@@ -107,7 +107,7 @@ def to_python(self, value):
             self.name,
             return_unit,
         )
-        logger.warn(msg)
+        logger.warning(msg)
         return get_measurement(
             measure=self.measurement,
             value=value,