Bump black from 19.3b0 to 24.3.0 in /requirements

Bumps [black](https://github.com/psf/black) from 19.3b0 to 24.3.0.
- [Release notes](https://github.com/psf/black/releases)
- [Changelog](https://github.com/psf/black/blob/main/CHANGES.md)
- [Commits](https://github.com/psf/black/commits/24.3.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

requirements/testing.txt

@@ -1,6 +1,6 @@
 betamax-serializers==0.2.0
 betamax==0.8.1
-black==19.3b0; python_version >= '3.6'
+black==24.3.0; python_version >= '3.6'
 coverage==4.5.3
 flake8==3.7.7
 isort==4.3.20