build!: drop Python 3.6 support (#431)

* docs: update to Myst-NB configuration v0.14
  https://github.com/executablebooks/MyST-NB/blob/master/CHANGELOG.md#v0140---2022-04-27
* fix: write missing latex in analytic continuation notebook
* style: rewrite type hints with PEP563
  https://peps.python.org/pep-0563
* style: run pyupgrade for Python 3.7

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Author: 3 people