Make the ForeiginKey detection more accurate

To handle this case: the project use tastypie and django.
tastypie has a `ForeignKey` field which has the same name
as django's `ForeignKey`.
The issue is the lint trys resolving the `ForeignKey` for the
tastypie `ForeignKey` which cause import error.

In this commit, add a check to ensure the current class of the
`ForeignKey` is a subclass of `Model` of django.

Tested manually

Author: Tommy Wu

pylint_django/transforms/foreignkey.py

@@ -16,6 +16,15 @@ def is_foreignkey_in_class(node):
     if not isinstance(node.parent.parent, ClassDef):
         return False
 
+    is_in_django_model_class = False
+    # Make sure the class is the subclass of django.db.models.Model
+    for ancestor in node.parent.parent.ancestors():
+        if ancestor.name == 'Model':
+            is_in_django_model_class = True
+            break
+    if not is_in_django_model_class:
+        return False
+
     if isinstance(node.func, Attribute):
         attr = node.func.attrname
     elif isinstance(node.func, nodes.Name):