Update testtools requirement from >=2.4.0 to >=2.9.1

[dependabot]

Updates the requirements on testtools to permit the latest version.

Changelog

Sourced from testtools's changelog.

2.9.1

Improvements
------------
* Don't leak testtools-specific exception details into
  ``unittest.TestResult`` error reports when using a stdlib ``TestResult``.
  (Jelmer Vernooij, [#607](https://github.com/testing-cabal/testtools/issues/607))
2.9.0

Improvements

• Add LessThanOrEqual and GreaterThanOrEqual matchers. (Julian Edwards, #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](https://github.com/testing-cabal/testtools/issues/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, #578)

2.8.5

Improvements
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... (truncated)

Commits

• 088c98e release 2.9.1
• 5e44c18 Don't leak testtools internals into stdlib TestResult error reports
• 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
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