Myst-nb

✨ NEW: `nb_merge_streams` configuration [[PR 364](https://github.com/executablebooks/MyST-NB/pull/364)]

If `nb_merge_streams=True`, all stdout / stderr output streams are merged into single outputs. This ensures deterministic outputs.

which is detailed in the [myst-parser changelog](https://myst-parser.readthedocs.io/en/latest/develop/_changelog.html).

The primary change is to the extension system, with extensions now all loaded *via* `myst_enable_extensions = ["dollarmath", ...]`,
and a number of extensions added or improved.

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