Remove Python 3.6 support across our repositories

[choldgraf]

Description

Python has reached its end of life for security support, and this is currently the criteria we use to decide whether to support versions of Python. So, we can safely remove Python 3.6 support across our repositories. This is a meta-issue to track where this is happening, and to make it clear that this is a pre-approved decision across whatever EBP repository somebody wants to make this change 🎉

To remove 3.6 support for a package

These are some steps to take

• Remove from Python 36 references to any tox or nox files
• Remove 3.6 from our GitHub Action builds
• Remove 3.6 from any docs / READMEs
• Remove 3.6 from the package metadata (setup.py, setup.cfg, pyproject.toml)

Core repositories to track

We've got a bunch of repositories, but here's a subset of repositories that are needed as part of the default Jupyter Book installation.

if you know one of these repositories has already deprecated 3.6, feel free to just tick the box

• [ ] https://github.com/executablebooks/MyST-NB
• [x] https://github.com/executablebooks/markdown-it-py
• [ ] https://github.com/executablebooks/linkify-it-py
• [ ] https://github.com/executablebooks/sphinx-comments
• [ ] https://github.com/executablebooks/sphinx-external-toc
• [ ] https://github.com/executablebooks/sphinx-jupyterbook-latex
• [ ] https://github.com/executablebooks/sphinx-panels (should be updated to sphinx-design though)
• [ ] https://github.com/executablebooks/sphinx-thebe
• [ ] https://github.com/executablebooks/sphinx-togglebutton
• [ ] https://github.com/executablebooks/sphinx-multitoc-numbering
• [ ] https://github.com/executablebooks/myst-parser
• [ ] https://github.com/executablebooks/jupyter-book: https://github.com/executablebooks/jupyter-book/pull/1626
• [ ] https://github.com/executablebooks/jupyter-cache
• [ ] https://github.com/executablebooks/sphinx-book-theme
• [ ] https://github.com/executablebooks/sphinx-copybutton