Bump pygments from 2.3.1 to 2.7.4 in /docs

Bumps [pygments](https://github.com/pygments/pygments) from 2.3.1 to 2.7.4.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.3.1...2.7.4)

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

Author: dependabot[bot]

docs/requirements.txt

@@ -2,7 +2,7 @@
 # crate-docs-theme>=0.7
 okchain1-docs-theme>=0.1.24
 sphinx-csv-filter
-Pygments==2.3.1
+Pygments==2.7.4
 
 
 alabaster==0.7.12