decode base64 output string to UTF-8

Fix byte string in python 3

liquid_tags/graphviz.py

@@ -116,7 +116,7 @@ def graphviz_parser(preprocessor, tag, markup):
         output = run_graphviz(program, code)
 
         # Return Base64 encoded image
-        return '<div class="graphviz" style="text-align: center;"><img src="data:image/png;base64,%s"></div>' % base64.b64encode(output)
+        return '<div class="graphviz" style="text-align: center;"><img src="data:image/png;base64,%s"></div>' % base64.b64encode(output).decode('utf-8')
 
     else:
         raise ValueError('Error processing input. '