Even if that is the case, you are still in the wrong place here.
The fail2ban package is provided by your OS vendor and if this one does use python2, then you need to talk to CentOS, Ubuntu, Debian or whatever you are using.
If you are bold, you can try to change the shebang line in the file /usr/bin/fail2ban-server yourself (from #!/usr/bin/python2 to #!/usr/bin/python3) and see what happens
Of course this will be without any support from either Plesk nor your OS vendor and whenever there will be an update for the fail2ban package, this file and with it the shebang line, will get overwritten.