Fix SerializingHTMLBuilder has been crashed if jinja2 helper functions are installed.

sphinx/builders/html.py

@@ -13,6 +13,7 @@ import codecs
 import posixpath
 import re
 import sys
+import types
 import warnings
 from hashlib import md5
 from os import path
@@ -1410,6 +1411,11 @@ class SerializingHTMLBuilder(StandaloneHTMLBuilder):
         # actually rendered
         self.app.emit('html-page-context', pagename, templatename, ctx, event_arg)
 
+        # make context object serializable
+        for key in list(ctx):
+            if isinstance(ctx[key], types.FunctionType):
+                del ctx[key]
+
         ensuredir(path.dirname(outfilename))
         self.dump_context(ctx, outfilename)