Bump Python requirement python-gitlab to 3.6.0

requirements.txt

 arkindex-client==1.0.8
 peewee==3.14.10
 Pillow>=9.0
-python-gitlab==2.7.1
+python-gitlab==3.6.0
 python-gnupg==0.4.8
 sh==1.14.2
 shapely==1.8.2