Bump pygments from 2.3.1 to 2.6.1 in /cvat/requirements

Bumps [pygments](https://github.com/pygments/pygments) from 2.3.1 to 2.6.1.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.3.1...2.6.1)

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

cvat/requirements/base.txt

@@ -29,7 +29,7 @@ coreapi==2.3.3
 django-filter==2.0.0
 Markdown==3.2.2
 djangorestframework==3.9.3
-Pygments==2.3.1
+Pygments==2.6.1
 drf-yasg==1.17.0
 Shapely==1.6.4.post2
 pdf2image==1.6.0