Jupytext related metadata should go to a jupytext section

[mwouts]

Jupytext currently uses the following metadata:

• jupytext_formats to configure the desired notebook representations
• encoding and executable to represent encoding and executable information contained in scripts
• jupytext_format_version, in text files, to store the format version for the given file.

Most jupyter extensions store their metadata in a dedicated entry in the jupyter notebook to avoid conflits with other extensions. We should do the same.

[mwouts]

Available in v0.8.0