Myst-nb

Upgraded to `sphinx` v4 ⬆️

The primary change in this release is to update the requirements of myst-nb from `sphinx>=2,<4` to `sphinx>=3,<5` to
support `sphinx>=4` [[PR 356](https://github.com/executablebooks/MyST-NB/pull/356)].

- 👌 IMPROVE: Allows more complex suffixes in notebooks [[PR 328](https://github.com/executablebooks/MyST-NB/pull/328)]
- ⬆️ UPDATE: myst-parser to `0.15.2` [[PR 353](https://github.com/executablebooks/MyST-NB/pull/353)]
- ⬆️ UPGRADE: nbconvert 6 support [[PR 326](https://github.com/executablebooks/MyST-NB/pull/326)]
- ⬆️ UPGRADE: markdown-it-py v1.0 [[PR 320](https://github.com/executablebooks/MyST-NB/pull/320)]
- 🔧 MAINT: Pin ipykernel to ~v5.5 [[PR 347](https://github.com/executablebooks/MyST-NB/pull/347)]
- 🔧 MAINT: Make a more specific selector for no-border [[PR 344](https://github.com/executablebooks/MyST-NB/pull/344)]

Many thanks to akhmerov, bollwyvl, choldgraf, chrisjsewell, juhuebner, mmcky

: Minor bug fixes and enhancements / new features

- ⬆️ UPDATE: jupyter_sphinx to `0.3.2`: fixes `Notebook code has no file extension metadata` warning)
- ⬆️ UPDATE: importlib_metadata to `3.6`: to use new entry point loading interface
- Official support for Python 3.9

This release adds an experimental MyST-NB feature to enable loading of code from a file
for `code-cell` directives using a `:load: <file>` option.

Usage information is available in the [docs](https://myst-nb.readthedocs.io/en/latest/use/markdown.html#syntax-for-code-cells)

Minor update to handle MyST-Parser `v0.13.3` and `v4.5` notebooks.