Unevaluated |default_python| in docs

[astrojuanlu]

After my Binder builds started failing because my project dropped support for Python 3.7, I tried to locate in the Binder docs what is the default Python version, and found this:

https://mybinder.readthedocs.io/en/latest/howto/languages.html#python

It took me a few minutes to figure out that this file belongs to repo2docker:

https://github.com/jupyterhub/repo2docker/blob/47ea95a74328b10fe0725cbd303ad4ae45c47c79/docs/source/howto/languages.rst?plain=1#L19-L21

and that there is some logic that fills that variable:

https://github.com/jupyterhub/repo2docker/blob/a07fe38f77aaa993398ff7b0b9a7f4ecbfcb886f/docs/source/conf.py#L49

However, it's not clear to me why it is not working.