Python: Fix ExceptStmt::getType

We were not supporting `except` statements handling multiple exception
types (specified as a tuple) correctly, instead just returning the
tuple itself as the "type" (which makes little sense).

To fix this, we explicitly extract the elements of this node, in the
case where it _is_ a tuple.

This is a change that can potentially affect many queries (as `getType`
is used in quite a few places), so some care should be taken to
ensure that this does not adversely affect performance.

python/ql/src/semmle/python/Stmts.qll

@@ -153,6 +153,12 @@ class ExceptStmt extends ExceptStmt_ {
   override Stmt getASubStatement() { result = this.getAStmt() }
 
   override Stmt getLastStatement() { result = this.getBody().getLastItem().getLastStatement() }
+
+  override Expr getType() {
+    result = super.getType() and not result instanceof Tuple
+    or
+    result = super.getType().(Tuple).getAnElt()
+  }
 }
 
 /** An assert statement, such as `assert a == b, "A is not equal to b"` */

python/ql/test/query-tests/Exceptions/generators/UnguardedNextInGenerator.expected

@@ -1,3 +1,2 @@
 | test.py:5:15:5:22 | ControlFlowNode for next() | Call to next() in a generator |
 | test.py:10:20:10:27 | ControlFlowNode for next() | Call to next() in a generator |
-| test.py:62:19:62:28 | ControlFlowNode for next() | Call to next() in a generator |