fix ISO8601 re

closes  #4493

IPython/utils/jsonutil.py

@@ -34,7 +34,7 b" next_attr_name = '__next__' if py3compat.PY3 else 'next'"
 
 # timestamp formats
 ISO8601="%Y-%m-%dT%H:%M:%S.%f"
-ISO8601_PAT=re.compile(r"^(\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d+)Z?([\+\-]\d{2}:?\d{2})?$")
+ISO8601_PAT=re.compile(r"^(\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{1,6})Z?([\+\-]\d{2}:?\d{2})?$")
 
 #-----------------------------------------------------------------------------
 # Classes and functions