Update testtools requirement from >=2.4.0 to >=2.9.0

[dependabot]

Updates the requirements on testtools to permit the latest version.

Changelog

Sourced from testtools's changelog.

2.9.0

Improvements
------------
* Add ``LessThanOrEqual`` and ``GreaterThanOrEqual`` matchers.
  (Julian Edwards, [#594](https://github.com/testing-cabal/testtools/issues/594))

Improve type annotations throughout the codebase, including compatibility
fixes for mypy 1.20.0.
(Stephen Finucane, #587, #591, #592, #593, #600)

2.8.7

Improvements

• Fix an issue with iterate_tests, which was recently changes to insist on accepting an instance of unittest.TestSuite, breaking some users. (Jelmer Vernooij, #584)

• Improve the type annotations for testtools.testcase.TestCase.skipTest (Stephen Finucane, #585)

• Correct a URL in the README (Steve Kowalik, #582)

2.8.6

Improvements
------------
* Improve the type annotations for ``testtools.testcase.TestCase.assertRaises``.
  This method will now be typed correctly when called as a method or as a
  context manager.
  (Stephen Finucane, [#577](https://github.com/testing-cabal/testtools/issues/577), [#578](https://github.com/testing-cabal/testtools/issues/578))
2.8.5

Improvements

• Fix the type annotations for testtools.testcase.TestCase.useFixture (Stephen Finucane, #574)

2.8.4

Improvements
------------
</tr></table> 

... (truncated)

Commits

• 18b9d32 Merge pull request #603 from stephenfin/release-2.9.0
• 729f33c release 2.9.0
• 18dac2f Merge pull request #602 from testing-cabal/dependabot/pip/ruff-0.15.9
• d289f82 build(deps-dev): bump ruff from 0.15.8 to 0.15.9
• 2c9f793 Merge pull request #600 from stephenfin/typing
• ec15b81 typing: Make less use of ParamSpec
• 231d06d typing: Fix compatibility with mypy 1.20.0
• 08ec4e6 Merge pull request #599 from testing-cabal/dependabot/pip/ruff-0.15.8
• 692185c build(deps-dev): bump ruff from 0.15.7 to 0.15.8
• d1bf6d5 Merge pull request #598 from testing-cabal/issue/594
• Additional commits viewable in compare view

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[dependabot]

Superseded by #1223.