End of Life for Python=3.6

[mmcky]

@chrisjsewell had posted this link at provides a nice calendar of python end of life dates

https://devguide.python.org/#status-of-python-branches

We should remove python=3.6 across all repos in January, 2022

• myst-parser
• myst-nb
• jupyter-book

[choldgraf]

I'd actually be a fan of something slightly more aggressive, which would be to only "officially" support the 3 last major releases of Python. So right now we'd support python 3.9/8/7. 3.6 would already no longer be "official"

[mmcky]

I can get on board with that :-)