This port will be removed on 2020-12-31 if not fixed
This is used by https://svnweb.freebsd.org/
There is some work upstream on https://github.com/viewvc/viewvc
However, until now there is no new release supporting python3.
I can only suggest to look into a different solution like websvn-php or usvn since I do not belive there will be a release before python2 is removed from ports