dns/knot2: Update to 2.9.6

Changelog: https://www.knot-dns.cz/2020-08-31-version-296.html

Remove Python from DOCS option, it is not necessary at this moment. [1]

PR: 249169
PR: 243374 [1]
Submitted by: Leo Vandewoestijne <freebsd@dns.company> (maintainer)
Reported by: alex_y_xu@yahoo.ca [1]