Replace <> with != in citation filter (invalid Py3 syntax)

IPython/nbconvert/filters/citation.py

@@ -45,7 +45,7 def citation2latex(s):
             outtext += s[startpos:citation[1]]
             outtext += '\\cite{%s}'%citation[0]
             startpos = citation[2] if len(citation)==3 else -1
-    outtext += s[startpos:] if startpos <> -1 else ''
+    outtext += s[startpos:] if startpos != -1 else ''
     return outtext
 
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