Import HTMLParser from html.parser for Python 3.x

IPython/nbconvert/filters/citation.py

@@ -11,7 +11,11 b''
 #-----------------------------------------------------------------------------
 # Imports
 #-----------------------------------------------------------------------------
-from HTMLParser import HTMLParser
+from IPython.utils.py3compat import PY3
+if PY3:
+    from html.parser import HTMLParser
+else:
+    from HTMLParser import HTMLParser
 
 #-----------------------------------------------------------------------------
 # Functions