Suppress no-member errors for RelatedManager fields

seen in a real project where we have `product.build.first()` where
Build.product is a FK to Product with related_name=build all of
which causes factory to build a RelatedManager on the fly.

For some reason I can't reproduce with the existing test but when
I make this change it resolves the problem in the original code base
that I was inspecting.

Author: atodorov

pylint_django/augmentations/__init__.py

@@ -507,6 +507,7 @@ def is_manager_attribute(node):
     """Checks that node is attribute of Manager or QuerySet class."""
     parents = ('django.db.models.manager.Manager',
                '.Manager',
+               'factory.base.BaseFactory.build',
                'django.db.models.query.QuerySet',
                '.QuerySet')
     return _attribute_is_magic(node, MANAGER_ATTRS.union(QS_ATTRS), parents)

pylint_django/tests/input/external_factory_boy_noerror.py

@@ -15,7 +15,7 @@ class Author(models.Model):
 
 class Book(models.Model):
     title = models.CharField()
-    author = models.ForeignKey(Author, on_delete=models.CASCADE)
+    author = models.ForeignKey(Author, related_name='books', on_delete=models.CASCADE)
 
 
 class AuthorFactory(factory.django.DjangoModelFactory):
@@ -40,6 +40,9 @@ class BookTestCase(test.LiveServerTestCase):
     def _fixture_setup(self):
         super(BookTestCase, self)._fixture_setup()
         self.book = BookFactory()
+        _author = AuthorFactory()
+        _first_book = _author.books.first()
+        self.assertIsNotNone(_first_book)
 
     def test_author_is_not_none(self):
         self.assertGreater(self.book.pk, 0)