Bump pygments from 2.7.4 to 2.15.0 in /django

Bumps [pygments](https://github.com/pygments/pygments) from 2.7.4 to 2.15.0.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.7.4...2.15.0)

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

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

django/requirements-doc.txt

@@ -2,7 +2,7 @@ coreapi==2.3.3
 coreapi-cli==1.0.9
 docutils==0.13.1
 Markdown==2.6.9
-Pygments==2.7.4
+Pygments==2.15.0
 Sphinx==2.4.2
 sphinxcontrib-images==0.9.1
 watchdog[watchmedo]==0.10.2