Bump pygments from 2.7.2 to 2.7.3 (#5318)

Bumps [pygments](https://github.com/pygments/pygments) from 2.7.2 to 2.7.3.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.7.2...2.7.3)

Signed-off-by: dependabot[bot] <[email protected]>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements/doc.txt

@@ -1,5 +1,5 @@
 aiohttp-theme==0.1.6
-pygments==2.7.2
+pygments==2.7.3
 sphinx==3.3.1
 sphinxcontrib-asyncio==0.3.0
 sphinxcontrib-blockdiag==2.0.0