pygments library version (material / mkdocs dependency)

Changes suggested by GitHub dependabot.  This is a "vulnerability" but minor in our case.  Fixing to silence this warning.

Author: normanlorrain

requirements.in

@@ -4,3 +4,5 @@ mkdocs-material>=9.7.0
 
 # GitHub's dependabot detected an error in previous versions
 urllib3>=2.6.0
+
+pygments~=2.20.0

requirements.txt

@@ -49,8 +49,10 @@ pathspec==0.12.1
     # via mkdocs
 platformdirs==4.5.1
     # via mkdocs-get-deps
-pygments==2.19.2
-    # via mkdocs-material
+pygments==2.20.0
+    # via
+    #   -r requirements.in
+    #   mkdocs-material
 pymdown-extensions==10.18
     # via mkdocs-material
 python-dateutil==2.9.0.post0