Fixed #19908 -- Added missing import in forms.py

Thanks loic84 for the report.

1 files changed, 1 insertions, 0 deletions

diff --git a/django/forms/forms.py b/django/forms/forms.py
index dccb12657e..20a2e426bb 100644
--- a/django/forms/forms.py
+++ b/django/forms/forms.py
@@ -5,6 +5,7 @@ Form classes
 from __future__ import absolute_import, unicode_literals
 
 import copy
+import warnings
 
 from django.core.exceptions import ValidationError
 from django.forms.fields import Field, FileField