Move away from deprecated/unmaintained sphinx_testing

[thesamesam]

Describe the bug

context
When trying to mark support for Python 3.11 to sphinx-tabs in our Gentoo packaging, we had to stop because one of its test dependencies, sphinx_testing, fails tests with Python 3.11 and is now archived upstream & unmaintained.

expectation
I expected all test dependencies to support Python 3.11.

bug

sphinx-tabs relies on an unmaintained package (sphinx_testing) for tests which prevents Python 3.11 compatibility.

problem
This is a problem for people migrating their systems and distributions to Python 3.11 because they can't test sphinx-tabs correctly first, which is often a blocker for making the migration at all (if it can't be tested, people don't feel comfortable using it with Python 3.11, so the package is usually removed instead).

Reproduce the bug

1. Install Python 3.11.
2. Try to run the sphinx-tabs test suite.

List your environment

• Python 3.11
• Gentoo

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[thesamesam]

This came up at #126 (comment) before too.

[foster999]

Removed this dependency in the latest release