Seafile-pro-server-9.0.14 install error 'No module named 'pymysql.constants'

Please can anybody help with this Problem: I try to install seafile server 9.0.14 with install script on Ubuntu 20.04. Environment: Python 3.8.10, mySQL 8.0.32, /usr/bin/python3 in $PATH.
./ ends with this message:
Checking python on this machine …
Checking for java …Done.
Traceback (most recent call last):

  • File “”, line 19, in *
  •  import pymysql*
  • File “/home/seafile/seafile-pro-server-9.0.14/seahub/thirdpart/pymysql/”, line 26, in *
  • from .constants import FIELD_TYPE*
    ModuleNotFoundError: No module named ‘pymysql.constants’

Thanks for any help!