fix: requirements.txt to reduce vulnerabilities


The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-PYGMENTS-1086606

requirements.txt

diff --git a/requirements.txt b/requirements.txt
index e9489d5..ddc95cc 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,2 +1,3 @@
 Sphinx>=1.6
 pyparsing
+pygments>=2.7.4 # not directly required, pinned by Snyk to avoid a vulnerability