[1.7.x] Fixed #23594 -- Fixed deepcopy on ErrorList.

Thanks Troy Grosfield for the report and Tim Graham for the tests.

Backport of ec2fd02bb3 from master

tests/forms_tests/tests/test_util.py

@@ -1,6 +1,8 @@
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 
+import copy
+
 from django.core.exceptions import ValidationError
 from django.forms.utils import flatatt, ErrorDict, ErrorList
 from django.test import TestCase
@@ -66,3 +68,24 @@ class FormsUtilTestCase(TestCase):
                          '<ul class="errorlist"><li>nameExample of link: &lt;a href=&quot;http://www.example.com/&quot;&gt;example&lt;/a&gt;</li></ul>')
         self.assertHTMLEqual(str(ErrorDict({'name': mark_safe(example)})),
                          '<ul class="errorlist"><li>nameExample of link: <a href="http://www.example.com/">example</a></li></ul>')
+
+    def test_error_dict_copy(self):
+        e = ErrorDict()
+        e['__all__'] = ErrorList([
+            ValidationError(
+                message='message %(i)s',
+                params={'i': 1},
+            ),
+            ValidationError(
+                message='message %(i)s',
+                params={'i': 2},
+            ),
+        ])
+
+        e_copy = copy.copy(e)
+        self.assertEqual(e, e_copy)
+        self.assertEqual(e.as_data(), e_copy.as_data())
+
+        e_deepcopy = copy.deepcopy(e)
+        self.assertEqual(e, e_deepcopy)
+        self.assertEqual(e.as_data(), e_copy.as_data())