PY-18634: Support form-based views

* form_valid(form) and form_invalid(form) methods of FormView have "form" argument of type that depends on form_class attribute of FormView class.

python/psi-api/src/com/jetbrains/python/psi/types/PyTypeProviderBase.java

@@ -60,6 +60,7 @@ public class PyTypeProviderBase implements PyTypeProvider {
   }
 
   @Override
+  @Nullable
   public Ref<PyType> getParameterType(@NotNull PyNamedParameter param, @NotNull PyFunction func, @NotNull TypeEvalContext context) {
     return null;
   }