After Upgrade to Seafile 9: cannot start any longer: ImportError: cannot import name 'python_2_unicode_compatible'

Hi,
I am running seafile server on ubuntu linux 20.04. After upgrade from seafile 8 to seafile 9 today I cannot start seahub any longer. The following error message is thrown during startup:

Traceback (most recent call last):
File “/home/sfadmin/sfroot/seafile-server-9.0.4/seahub/manage.py”, line 10, in
execute_from_command_line(sys.argv)
File “/home/sfadmin/sfroot/seafile-server-9.0.4/seahub/thirdpart/django/core/management/init.py”,
line 419, in execute_from_command_line
utility.execute()
File “/home/sfadmin/sfroot/seafile-server-9.0.4/seahub/thirdpart/django/core/management/init.py”,
line 395, in execute
django.setup()
File “/home/sfadmin/sfroot/seafile-server-9.0.4/seahub/thirdpart/django/init.py”, line 24, in setu
p
apps.populate(settings.INSTALLED_APPS)
File “/home/sfadmin/sfroot/seafile-server-9.0.4/seahub/thirdpart/django/apps/registry.py”, line 114, i
n populate
app_config.import_models()
File “/home/sfadmin/sfroot/seafile-server-9.0.4/seahub/thirdpart/django/apps/config.py”, line 301, in
import_models
self.models_module = import_module(models_module_name)
File “/usr/lib/python3.8/importlib/init.py”, line 127, in import_module
return _bootstrap._gcd_import(name[level:], package, level)
File “”, line 1014, in _gcd_import
File “”, line 991, in _find_and_load
File “”, line 975, in _find_and_load_unlocked
File “”, line 671, in _load_unlocked
File “”, line 848, in exec_module
File “”, line 219, in _call_with_frames_removed
File “/usr/lib/python3/dist-packages/captcha/models.py”, line 4, in
from django.utils.encoding import python_2_unicode_compatible
ImportError: cannot import name ‘python_2_unicode_compatible’ from ‘django.utils.encoding’ (/home/sfadmi
n/sfroot/seafile-server-9.0.4/seahub/thirdpart/django/utils/encoding.py)

I had the same error message. The terminal command

pip install captcha
(or: pip3 install captcha)

solved this problem for me. Afterwards, the ./seahub.sh start-fastcgi does not work anymore, but starting seahub the regular way, that is with ./start.sh, does.