Bump Python requirement python-gitlab to 3.15.0

requirements.txt

@@ -2,7 +2,7 @@ arkindex-client==1.0.12
 peewee==3.16.2
 Pillow==9.5.0
 pymdown-extensions==10.0.1
-python-gitlab==3.14.0
+python-gitlab==3.15.0
 python-gnupg==0.5.0
 sh==2.0.4
 shapely==2.0.1