if it could be useful for someone else, even after following all steps here https://download.seafile.com/published/seafile-manual/upgrade/upgrade_notes_for_7.1.x.md Seahub was not starting. Starting manually throw this error:
File "/opt/seafile/seafile-server-7.1.3/seahub/thirdpart/django/core/cache/backends/memcached.py", line 162, in __init__ import memcache ModuleNotFoundError: No module named 'memcache'
To make it working I had to install also python3 memcached with command
pip3 install --timeout=3600 python-memcached
Maybe documentation should be updated?