dev-python/nbval: new package.

dev-python/nbval: new package.

nbval is a py.test plugin to validate Jupyter notebook.

The plugin adds functionality to py.test to recognise and collect Jupyter
notebooks.  The intended purpose of the tests is to determine whether execution
of the stored inputs match the stored outputs of the .ipynb file. Whilst also
ensuring that the notebooks are running without errors.

The tests were designed to ensure that Jupyter notebooks (especially those for
reference and documentation), are executing consistently.

Each cell is taken as a test, a cell that doesn't reproduce the expected output
will fail.

Closes: https://bugs.gentoo.org/638208
Closes: https://github.com/gentoo/gentoo/pull/6247
Package-Manager: Portage-2.3.13, Repoman-2.3.3