⬆️ Update pygments requirement from <2.20 to <2.21

Updates the requirements on [pygments](https://github.com/pygments/pygments) to permit the latest version.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@0.5...2.20.0)

---
updated-dependencies:
- dependency-name: pygments
  dependency-version: 2.20.0
  dependency-type: direct:development
...

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

Author: dependabot[bot]

pyproject.toml

@@ -75,7 +75,7 @@ testing = [
     "pytest-regressions",
     "pytest-param-files~=0.6.0",
     "sphinx-pytest~=0.3.0",
-    "pygments<2.20",  # TODO fix test regression with 2.19"
+    "pygments<2.21",  # TODO fix test regression with 2.19"
 ]
 testing-docutils = [
     "pygments",