Support Python 3.10

[choldgraf]

Description / Summary

We are not currently testing or officially supporting Python 3.10 in Jupyter Book.
We should at least:

• Start testing Python 3.10 in our test suite
• Make sure our tests pass
• Update our metadata to support the latest 3 Python versions (3.10/9/8)

We should do this after #1448 is done to reduce any potential Sphinx 2.x bugs we might hit

(originally reported in #1490)

Value / benefit

More and more people will use 3.10 over time, so we might start getting unexpected errors once this happens.

Implementation details

Metadata about python version is here: https://github.com/executablebooks/jupyter-book/blob/master/setup.cfg

Our test matrix is here:

jupyter-book/.github/workflows/tests.yml

Lines 81 to 90 in a0d719f

	tests:
	runs-on: ubuntu-latest
	strategy:
	matrix:
	python-version: [3.6, 3.7]
	sphinx: [">=3,<4"]
	include:
	- python-version: 3.8
	sphinx: ">=2,<3"

(not sure why we only test 3.8 with Sphinx 2/3 but we might as well try doing a full matrix unless we know there are incompatible versions matches)

This PR attempted to do this, but upgrading to Python 3.10 was not a super quick fix: #1509

Issues we ran into:

• Python 3.10 isn't supported in the version of pytest we use. Upgrading to the latest pytest results in some errors in our builds, so we'll need to debug that.

Tasks to complete

• IMPROVE: Support Sphinx v4  #1448
• 🚀 RELEASE: Setup and Changes for v0.12 #1494
• MAINT: Unpin max version for pytest #1510
• Implement the changes described above and see if tests pass

[chrisjsewell]

Update our metadata to support the latest 3 Python versions (3.10/9/8)

This is not the correct way to support python. They should be supported by their end of life dates https://devguide.python.org/#status-of-python-branches

[choldgraf]

ah very helpful list! that sounds reasonable to me - I suppose we just need to add 3.9/3.10 to the tests, and then test them as well. Is it reasonable to test the latest 3 python versions, even though we technically support versions that are earlier than those three?

I also opened up executablebooks/meta#513 to document this policy, since we might as well apply it across the other tools in the stack