Backport PR #6878: Update pygments monkeypatch for compatibility with Pygments 2.0

Closes gh-6877

I spent a while looking for a better way to achive this, to get rid of the monkeypatch, but none occurred to me, so I went with Carlos' suggestion of just updating it.

IPython/qt/console/pygments_highlighter.py

@@ -12,6 +12,11 b" def get_tokens_unprocessed(self, text, stack=('root',)):"
     """ Split ``text`` into (tokentype, text) pairs.
 
         Monkeypatched to store the final stack on the object itself.
+
+        The `text` parameter this gets passed is only the current line, so to
+        highlight things like multiline strings correctly, we need to retrieve
+        the state from the previous line (this is done in PygmentsHighlighter,
+        below), and use it to continue processing the current line.
     """
     pos = 0
     tokendefs = self._tokens
@@ -24,6 +29,7 b" def get_tokens_unprocessed(self, text, stack=('root',)):"
         for rexmatch, action, new_state in statetokens:
             m = rexmatch(text, pos)
             if m:
+                if action is not None:
                 if type(action) is _TokenType:
                     yield pos, action, m.group()
                 else: