BREAKING CHANGE: drop support for python 3.7

[aucampia]

Summary of changes

Python 3.7 will be end-of-life on the 27th of June 2023 and the next release of RDFLib will be a new major version.

This changes the minimum supported version of Python to 3.8.1 as some of the dependencies we use are not too fond of python 3.8.0. This change also removes all accommodations for older python versions.

• Closes Drop python 3.7 support #2432.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• If the change has a potential impact on users of this project:
  • Added or updated tests that fail without the change.
  • Updated relevant documentation to avoid inaccuracies.
  • Considered adding additional documentation.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

coverage: 90.882% (+0.03%) from 90.854% when pulling 1b23234 on aucampia:aucampia/20230610T1324-upgrade-python into d21a451 on RDFLib:main.

[ghost]

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