Fix undefined variable 'warnings'

cgohlke · web-flow · commit ab3dd1a4ad24 · 2017-10-16T11:22:30.000-07:00
diff --git a/lib/matplotlib/collections.py b/lib/matplotlib/collections.py
@@ -11,6 +11,8 @@
 from __future__ import (absolute_import, division, print_function,
                         unicode_literals)
 
+import warnings
+
 import six
 from six.moves import zip
 try:
