When I follow this I get an error as follows
Traceback (most recent call last):
File "/usr/libexec/postfix/policyd-spf", line 35, in <module>
import spf
ImportError: No module named 'spf'
(As a work around until I get this fixed in master.cf I use: user=nobody argv=/usr/bin/python /usr/libexec/postfix/policyd-spf
instead of: user=nobody argv=/usr/bin/python3 /usr/libexec/postfix/policyd-spf
Also I have to disable the ipaddress modules in python. While this causes other problems the server can at least receive mails)