Sphinx 编译报错nonlocal open_cite_tag

线上自动编译时出现错误 SyntaxError: invalid syntax
谷歌上搜 nonlocal open_cite_tag 没有任何相关信息

Running Sphinx v1.7.9
loading translations [zh_CN]... done

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/rqalpha/envs/develop/local/lib/python2.7/site-packages/sphinx/cmdline.py", line 303, in main
    args.warningiserror, args.tags, args.verbosity, args.jobs)
  File "/home/docs/checkouts/readthedocs.org/user_builds/rqalpha/envs/develop/local/lib/python2.7/site-packages/sphinx/application.py", line 191, in __init__
    self.setup_extension(extension)
  File "/home/docs/checkouts/readthedocs.org/user_builds/rqalpha/envs/develop/local/lib/python2.7/site-packages/sphinx/application.py", line 411, in setup_extension
    self.registry.load_extension(self, extname)
  File "/home/docs/checkouts/readthedocs.org/user_builds/rqalpha/envs/develop/local/lib/python2.7/site-packages/sphinx/registry.py", line 315, in load_extension
    mod = __import__(extname, None, None, ['setup'])
  File "/home/docs/checkouts/readthedocs.org/user_builds/rqalpha/envs/develop/local/lib/python2.7/site-packages/nbsphinx.py", line 1088
    nonlocal open_cite_tag
                         ^
SyntaxError: invalid syntax

Exception occurred:
  File "/home/docs/checkouts/readthedocs.org/user_builds/rqalpha/envs/develop/local/lib/python2.7/site-packages/sphinx/registry.py", line 315, in load_extension
    mod = __import__(extname, None, None, ['setup'])
  File "/home/docs/checkouts/readthedocs.org/user_builds/rqalpha/envs/develop/local/lib/python2.7/site-packages/nbsphinx.py", line 1088
    nonlocal open_cite_tag
                         ^
SyntaxError: invalid syntax
The full traceback has been saved in /tmp/sphinx-err-cnpEVe.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at . Thanks!

对照后发现 nbsphinx ==0.3.5时会成功
requirements.txt 文件加上版本限制就ok

• cat requirements.txt

Sphinx
watchdog
sphinx_rtd_theme
nbsphinx ==0.3.5
jupyter_client

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