[Samba] ImportError: No module named 'dns'

[Roy Eastwood]

After updating my samba version to 4.10.3 (from a self-compiled 4.10.2
version) on a DC running on Debian Stretch, I notice errors in the logs when
samba_dnsupdate is run.   Manually running it produces:

root at debian-vb:~# samba_dnsupdate --verbose
Traceback (most recent call last):
  File "/usr/local/samba/sbin/samba_dnsupdate", line 56, in <module>
    import dns.resolver
ImportError: No module named 'dns'

This may have been present for some time, but only just noticed it.   I have
checked that python-dnspython is installed:

root at debian-vb:~# dpkg -l | grep python-dnspython
ii  python-dnspython                1.15.0-1+deb9u1                   all
DNS toolkit for Python

(The other DC in the domain (also updated to 4.10.3 running on rasbian
stretch) works OK.)

It was working before, obviously something has changed.   How to fix?    Any
advice appreciated.   Let me know if you need more info.

Thanks,

Roy