chore(deps): update black requirement from ==25.* to ==26.*

Updates the requirements on [black](https://github.com/psf/black) to permit the latest version.
- [Release notes](https://github.com/psf/black/releases)
- [Changelog](https://github.com/psf/black/blob/main/CHANGES.md)
- [Commits](psf/black@25.1.0...26.5.1)

---
updated-dependencies:
- dependency-name: black
  dependency-version: 26.5.1
  dependency-type: direct:production
...

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

Author: dependabot[bot]

requirements-testing.txt

@@ -5,7 +5,7 @@ pytest == 8.4.*; python_version < '3.10'
 pytest-cov == 7.1.*
 pytest-timeout == 2.4.*
 pytest-xdist == 3.8.*
-black == 25.*
+black == 26.*
 ruff == 0.15.*
 mypy == 1.19.*
 psutil == 7.2.*