Update pytest requirement from >=8.0.0 to >=9.1.1

Updates the requirements on [pytest](https://github.com/pytest-dev/pytest) to permit the latest version.
- [Release notes](https://github.com/pytest-dev/pytest/releases)
- [Changelog](https://github.com/pytest-dev/pytest/blob/main/CHANGELOG.rst)
- [Commits](pytest-dev/pytest@8.0.0...9.1.1)

---
updated-dependencies:
- dependency-name: pytest
  dependency-version: 9.1.1
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements.txt

@@ -1,5 +1,5 @@
 flit>=3.12.0
 tox>=4.56.1
-pytest>=8.0.0
+pytest>=9.1.1
 mdformat >=0.7.0
 mdit_py_plugins >=0.4.0