Python: Add declares/getAttribute API

These could arguably be moved to `Class` itself, but for now I'm
choosing to limit the changes to the `DuckTyping` module (until we
decide on a proper API).

Author: tausbn

python/ql/lib/semmle/python/dataflow/new/internal/DataFlowDispatch.qll

@@ -1999,6 +1999,25 @@ module DuckTyping {
     hasMethod(cls, "__delete__")
   }
 
+  /**
+   * Holds if `cls` directly assigns to an attribute named `name` in its class body.
+   * This covers attribute assignments like `x = value`, but not method definitions.
+   */
+  predicate declaresAttribute(Class cls, string name) {
+    exists(getAnAttributeValue(cls, name))
+  }
+
+  /**
+   * Gets the value expression assigned to attribute `name` directly in the class body of `cls`.
+   */
+  Expr getAnAttributeValue(Class cls, string name) {
+    exists(Assign a |
+      a.getScope() = cls and
+      a.getATarget().(Name).getId() = name and
+      result = a.getValue()
+    )
+  }
+
   /**
    * Holds if `cls` is callable, i.e. it declares `__call__`.
    */