Feature: add support for .py files in py:percent format

[lukastk]

• Introduces the ability to handle .py files as py:percent files within nbdev.
  • Does this by converting .py files to .ipynb format as an intermediate step in NBProcessor.__init__
• Added example of usage in 'nbs/api/04_export.ipynb'.

Can export all *.py files by running nbdev_export --file_glob '*.py'

See https://jupytext.readthedocs.io/en/latest/formats-scripts.html for info on the py:percent format.

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[lukastk]

Related issue: https://github.com/fastai/nbdev/issues/1437

[jph00]

This is a great feature! But I don't want to add these additional deps to nbdev. Could we make it an extension or some other kind of separate project? Or alternatively only enable these new features if these deps are installed (but not make them requirements)?