dns/py-dns-lexicon: Update to 2.1.7

Changes: https://github.com/AnalogJ/lexicon/releases/tag/v2.1.7

PR: 220007
Submitted by: Goran Mekić <meka@tilda.center> (maintainer)
Approved by: garga (mentor)
Differential Revision: https://reviews.freebsd.org/D11220