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
Test case added: func_noerror_foreign_key_in_non_django_class

Author: Tommy Wu

Diff for: pylint_django/tests/input/external_tastypie_noerror_foreign_key.py

@@ -0,0 +1,21 @@
+"""
+Checks that Pylint doesn't raise an error when a 'ForeignKey' appears in a
+non-django class
+
+The real case is described as follow:
+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.
+"""
+from tastypie import fields
+from tastypie.fields import ForeignKey
+
+# pylint: disable=missing-docstring
+from tastypie.resources import ModelResource
+
+
+class MyTestResource(ModelResource):  # pylint: disable=too-few-public-methods
+    field1 = ForeignKey("myapp.api.resource", "xxx")
+    field2 = fields.ForeignKey("myapp.api.resource", "xxx")

Diff for: pylint_django/transforms/foreignkey.py

@@ -13,6 +13,11 @@ def is_foreignkey_in_class(node):
     if not isinstance(node.parent.parent, ClassDef):
         return False
 
+    # Make sure the outfit class is the subclass of django.db.models.Model
+    is_in_django_model_class = node_is_subclass(node.parent.parent, "django.db.models.base.Model", ".Model")
+    if not is_in_django_model_class:
+        return False
+
     if isinstance(node.func, Attribute):
         attr = node.func.attrname
     elif isinstance(node.func, nodes.Name):

Diff for: setup.py

@@ -23,7 +23,7 @@
     ],
     extras_require={
         "with_django": ["Django"],
-        "for_tests": ["django_tables2", "factory-boy", "coverage", "pytest", "wheel"],
+        "for_tests": ["django_tables2", "factory-boy", "coverage", "pytest", "wheel", "django-tastypie"],
     },
     license="GPLv2",
     classifiers=[