Myst-nb

Minor fixes:

- 🐛 FIX: empty myst file read
- 🐛 FIX: remove cell background-color CSS for cells
- 🔧 MAINTAIN: Pin jupyter-sphinx version

⬆️ UPGRADE: jupyter-sphinx v0.3, jupyter-cache v0.4.1 and nbclient v0.5.

: These upgrades allow for full Windows OS compatibility, and improve the stability of notebook execution on small machines.

👌 IMPROVE: Formatting of stderr is now similar to stdout, but with a slight red background.

🧪 TESTS: Add Windows CI

⬆️ UPGRADE: myst-parser patch version

: to ensure a few new features and bug fixes are incorporated (see its [CHANGELOG.md](https://github.com/executablebooks/MyST-Parser/blob/master/CHANGELOG.md))

More configuration!

- ✨ NEW: Add stderr global configuration: `nb_output_stderr`
(see [removing stderr](https://myst-nb.readthedocs.io/en/latest/use/formatting_outputs.html#removing-stdout-and-stderr))
- ✨ NEW: Add `nb_render_key` configuration
(see [formatting outputs](https://myst-nb.readthedocs.io/en/latest/use/formatting_outputs.html#images))
- 🐛 FIX: `auto` execution not recognising (and skipping) notebooks with existing outputs