Bump pygments from 2.3.1 to 2.15.0

Bumps [pygments](https://github.com/pygments/pygments) from 2.3.1 to 2.15.0.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.3.1...2.15.0)

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

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

Author: dependabot[bot]

requirements.txt

@@ -2,7 +2,7 @@ recommonmark==0.4.0
 sphinx-markdown-tables==0.0.9
 sphinx-rtd-theme==0.4.3
 
-Pygments==2.3.1
+Pygments==2.15.0
 sphinx==1.8.6
 Jinja2==2.11.3
 readthedocs-sphinx-ext==1.0.4