Merge pull request matplotlib#6030 from jenshnielsen/webagg_datatime_fix

Add missing datetime import to webagg

Author: pelson

lib/matplotlib/backends/backend_webagg_core.py

@@ -23,6 +23,7 @@
 
 import numpy as np
 import tornado
+import datetime
 
 from matplotlib.backends import backend_agg
 from matplotlib.figure import Figure