Bump Python requirement pymdown-extensions to 9.9

requirements.txt

 arkindex-client==1.0.11
 peewee==3.15.4
 Pillow==9.3.0
-pymdown-extensions==9.8
 python-gitlab==3.12.0
+pymdown-extensions==9.9
 python-gnupg==0.5.0
 sh==1.14.3
 shapely==1.8.5.post1