Python: Fix hasFlowPath default implementation of isSink/2

If hasFlowPath was used, and isSink/2 was not overridden,
hasFlowPath(src, sink) would not use isSink/1 to restrict the allowed TaintSink.
This resulted in false-positives when we had flows with unrelated TaintSinks.

FP: 1a8e7ffc2e/files/webapp/graphite/dashboard/views.py (x2d486922081db956):1

Fixes https://github.com/Semmle/ql/issues/2081

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

@@ -51,6 +51,7 @@ module TaintTracking {
          */
         predicate isSink(DataFlow::Node node, TaintKind kind) {
             exists(TaintSink sink |
+                this.isSink(sink) and
                 node.asCfgNode() = sink and
                 sink.sinks(kind)
             )