[1.7.x] Fixed #23492 -- Restored F.__deepcopy__.

This reverts commit 3a66035107.
A regression test was also added.

Backport of d63ac5b595 from master.

tests/expressions/tests.py

@@ -1,5 +1,7 @@
 from __future__ import unicode_literals
 
+from copy import deepcopy
+
 from django.core.exceptions import FieldError
 from django.db.models import F
 from django.db import transaction
@@ -281,3 +283,11 @@ class ExpressionsTests(TestCase):
             company_ceo_set__num_employees=F('company_ceo_set__num_employees')
         )
         self.assertEqual(str(qs.query).count('JOIN'), 2)
+
+    def test_F_object_deepcopy(self):
+        """
+        Make sure F objects can be deepcopied (#23492)
+        """
+        f = F("foo")
+        g = deepcopy(f)
+        self.assertEqual(f.name, g.name)