Python: Limit size of TupleElementContent

A more principled approach is possible here, but in the short term
this will prevent an explosion.

For reference, openstack/cinder has roughly 19000 `ForTarget`s and
tuples of size up to 5300, and we were calculating the cartesian
product of these.

python/ql/src/semmle/python/dataflow/new/internal/DataFlowPrivate.qll

@@ -1517,7 +1517,7 @@ predicate forReadStep(CfgNode nodeFrom, Content c, Node nodeTo) {
     or
     c instanceof SetElementContent
     or
-    c instanceof TupleElementContent
+    c.(TupleElementContent).getIndex() <= 7
   )
 }