Release v0.5 to PyPI

Thanks for this great project! However, when I did `pip install ipynb` I got version 0.4, which does not correctly comment out markdown cells when converting into code (fixed in commit b266e352bf1726477129270d4530ed2df7bdc580, which lies between 0.4 and 0.5). Currently one must do `pip install -e git+https://github.com/ipython/ipynb#egg=ipynb` to get the fixed version.

NB, the PyPI page also has an old project URL http://github.com/yuvipanda/ipynb instead of http://github.com/ipython/ipynb .

首先检查提交 b266e352bf1726479270d4530ed2df7bdc580 以及项目当前的软件包发布配置。将修正后的 v0.5 软件包发布到 PyPI，将 PyPI 项目 URL 更新为 ipython 仓库，并验证 pip install ipynb 能够获取修正后的版本。

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