Upgrade to Python 3 Flask 3

Resume some work found on to-python3 and new-isp-fields branches and make the project working

• Upgraded to Python 3.11
• Upgraded to Flask 3
• Working Docker compose environment

Identified work remaining :

• Some fields have been added (new-isp-fields), maybe it needs some data migration to ensure templates and data processing working as subscribers detail in each ISP technologies
• Test ISP edition
• Test production data migration / compatibility
• Add some documentation for the project (installation, end user guide)