Fix Python 2 compat: avoid 'nonlocal' statement

[pilou-]

Python 2 compatibility is broken since 0.4.0 / 46b3664.

avoid nonlocal statement: wrap open_cite_tag inside a mutable object.

$ python --version
Python 2.7.15+
$ python setup.py build_sphinx
running build_sphinx
Running Sphinx v1.8.3

Exception occurred:
  File "/tmp/nbsphinx.venv/local/lib/python2.7/site-packages/Sphinx-1.8.3-py2.7.egg/sphinx/registry.py", line 472, in load_extension
    mod = __import__(extname, None, None, ['setup'])
  File "/tmp/nbsphinx/src/nbsphinx.py", line 1088
    nonlocal open_cite_tag
                         ^
SyntaxError: invalid syntax

[mgeier]

Thanks for this PR!

Do you actually need Python 2 support?

I was thinking about removing Python 2 support, see also #268.

[pilou-]

Lago requires nbsphinx and is still not compatible with Python 3. While Lago could support Python 3 soon, I guess Lago won't drop Python 2 support.

[mgeier]

OK I've created #275.

I'm not using your PR because I don't like the "wrap it in a list" work-around for nonlocal. I prefer the "use a function attribute" work-around.

I can keep Python 2 compatibility for some more time, but at some point it will have to go away. In about a year the official support for Python 2 will end anyway ...

[mgeier]

I've merged #275.

[pilou-]

@mgeier thanks !

[jeffrey-hokanson]

Thanks for doing this. I've manually installed updated version, but the changes introduced in this merge aren't appearing in the latest pip version and, consequently, I'm having trouble using nbsphinx on readthedocs. Would a version bump fix this?

[mgeier]

@jeffrey-hokanson I haven't made a new release yet, but I will make one within the next few days.

In the meantime, if you are in a hurry, you can use the master branch of nbsphinx on readthedocs as described in the docs https://nbsphinx.readthedocs.io/en/0.4.1/usage.html#Automatic-Creation-of-HTML-and-PDF-output-on-readthedocs.org.

[mgeier]

I've just uploaded the new release 0.4.2 to PyPI.