Bump black in /packages/typespec-python/tests/requirements

Bumps [black](https://github.com/psf/black) from 24.8.0 to 26.3.1.
- [Release notes](https://github.com/psf/black/releases)
- [Changelog](https://github.com/psf/black/blob/main/CHANGES.md)
- [Commits](psf/black@24.8.0...26.3.1)

---
updated-dependencies:
- dependency-name: black
  dependency-version: 26.3.1
  dependency-type: direct:production
...

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

Author: dependabot[bot]

packages/typespec-python/tests/requirements/lint.txt

@@ -1,4 +1,4 @@
 # Linting dependencies
 -r base.txt
 pylint==4.0.4
-black==24.8.0
+black==26.3.1