GitHub Action: Add Python 3.10 and remove 3.6

[cclauss]

https://devguide.python.org/#status-of-python-branches vs. https://devguide.python.org/devcycle/#end-of-life-branches

All tests are green but .github/workflows/main.yml line 50 needs some attention from someone who knows coveralls.

• ~@graingert I took your changes from #253~ That did not work out. Reverted.

generator_stop is history in Python >= 3.7. https://docs.python.org/3/library/future.html

@takluyver @graingert Your reviews, please.