Bump pygments from 2.14.0 to 2.15.0 in /tests

Bumps [pygments](https://github.com/pygments/pygments) from 2.14.0 to 2.15.0.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.14.0...2.15.0)

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

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

tests/requirements.txt

@@ -5,7 +5,7 @@ coverage-pyver-pragma>=0.2.1
 domdf-python-tools[testing]>=2.0.1
 importlib-metadata>=3.6.0
 iniconfig!=1.1.0,>=1.0.1
-pygments==2.14.0
+pygments==2.15.0
 pytest>=6.0.0
 pytest-cov>=2.8.1
 pytest-randomly>=3.7.0