build: upgrade sphinx and change flake8 and related to only install on python >=3.8

[aucampia]

Summary of changes

The main aim of this change is to get to a newer version of Sphinx.

However, if just Sphinx is upgraded, poetry can't do version solving because of
the version of importlib-metadata that flake8 needs:

$ poetry install
Updating dependencies
Resolving dependencies... (0.6s)

Because no versions of flakeheaven match >3.2.1,<4.0.0
 and flakeheaven (3.2.1) depends on flake8 (>=4.0.1,<5.0.0), flakeheaven (>=3.2.1,<4.0.0) requires flake8 (>=4.0.1,<5.0.0).
And because no versions of flake8 match >4.0.1,<5.0.0
 and flake8 (4.0.1) depends on importlib-metadata (<4.3), flakeheaven (>=3.2.1,<4.0.0) requires importlib-metadata (<4.3).
And because sphinx (5.3.0) depends on importlib-metadata (>=4.8)
 and no versions of sphinx match >5.3.0,<6.0.0, flakeheaven (>=3.2.1,<4.0.0) is incompatible with sphinx (>=5.3.0,<6.0.0).
So, because rdflib depends on both sphinx (^5.3.0) and flakeheaven (^3.2.1), version solving failed.

To make things work, flake8 and related is only installed for Python >=3.8,
where the built-in importlib.metadata is used instead of the
importlib-metadata package.

This means no more flake8 on python 3.7, but it is a reasonable trade-off to get
to a newer version of Sphinx, and Python 3.7 support will be dropped by
2023-06-27 anyway.

Other changes:

• Changed Read the Docs to use the Sphinx version from poetry instead of the
  custom version that was in devtools/requirements-rtd.txt.
• Added typing-extensions to the poetry docs dependency group as it is
  needed for docs to build correctly.
• Changed the tox docs environment to be closer to the Read the Docs
  environment.

Closes #2272.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

Coverage: 90.782%. Remained the same when pulling 1624b72 on aucampia:iwana-20230318T1450-flake8_diff into 7a05c15 on RDFLib:main.

[aucampia]

Planning to merge this by 2023-03-21.

[ghost]

lgtm 👍

[aucampia]

Thanks for the review @gjhiggins