Add cmarkgfm to constraints.txt

The latest version of cmarkgfm breaks the build, see theacodes/cmarkgfm#84

Author: mkrnr

reqs/constraints.txt

@@ -12,6 +12,7 @@ charset-normalizer==3.4.1
 check-manifest==0.50
 click==8.1.8
 click-default-group==1.2.4
+cmarkgfm==2024.11.20
 colorama==0.4.6
 cryptography==44.0.2
 dmgbuild==1.6.5