Bump Python requirement python-gitlab to 2.7.1

requirements.txt

 arkindex-client==1.0.6
 peewee==3.14.4
 Pillow==8.2.0
-python-gitlab==2.6.0
+python-gitlab==2.7.1
 python-gnupg==0.4.7
 sh==1.14.2
 tenacity==7.0.0