Python: Handle taint for f-strings

Which we seem to not handle in the current taint tracking :O

f-strings needs to be Python 3 only, so enabled that test setup. I really liked
the idea for having the version specific tests right next to the normal tests,
so you don't have to look in
test/experimental/3/dataflow/i/will/forget/to/look/here.

python/ql/src/experimental/dataflow/internal/TaintTrackingPrivate.qll

@@ -125,6 +125,9 @@ predicate stringMethods(DataFlow::CfgNode nodeFrom, DataFlow::CfgNode nodeTo) {
     mult.getOp() instanceof Mult and
     mult.getLeft() = nodeFrom.getNode()
   )
+  or
+  // f-strings
+  nodeTo.getNode().getNode().(Fstring).getAValue() = nodeFrom.getNode().getNode()
   // TODO: Handle encode/decode from base64/quopri
   // TODO: Handle os.path.join
   // TODO: Handle functions in https://docs.python.org/3/library/binascii.html