Python: Only comparison with constant will clear taint

tainted = SOURCE
    if tainted == tainted:
        SINK(tainted) # unsafe

before, in the body of the if statement, `tainted` was not tainted

python/ql/src/semmle/python/dataflow/Implementation.qll

@@ -806,7 +806,10 @@ private predicate testEvaluatesMaybe(ControlFlowNode test, ControlFlowNode use)
     any(PyEdgeRefinement ref).getTest().getAChild*() = test and
     test.getAChild*() = use and
     not test.(UnaryExprNode).getNode().getOp() instanceof Not and
-    not Filters::equality_test(test, use, _, _) and
+    not exists(ControlFlowNode const |
+        Filters::equality_test(test, use, _, const) and
+        const.getNode() instanceof ImmutableLiteral
+    ) and
     not Filters::isinstance(test, _, use) and
     not test = use
     or