Python: Changing signature in overriden method is not an error

Rather, fulfiling the Liskov substitution principle is an opinionated
recommendation. Looking at `py/inheritance/incorrect-overridden-signature` and
`py/mixed-tuple-returns`, it seems very appropriate that this should have
`@severity recommendation`, and `@sub-severity high`.

python/ql/src/Functions/IncorrectlySpecifiedOverriddenMethod.ql

@@ -4,8 +4,8 @@
  *              the arguments with which it is called, and if it were called, would be likely to cause an error.
  * @kind problem
  * @tags maintainability
- * @problem.severity error
- * @sub-severity low
+ * @problem.severity recommendation
+ * @sub-severity high
  * @precision high
  * @id py/inheritance/incorrect-overridden-signature
  */