dns/py-dnspython: Revert 7fc16da

• Remove duplicate PORTVERSION based on PYTHON_MAJOR_VER
• Add PORTEPOCH as version number going backwards 2.1.0=>1.16.0
• Restrict python upto 3.7
• Fix finance/electrum which was changed based on this.
• Fix dns/py-dns-lexicon which was changed based on this.

PR: 261325
Approved by: portmgr
Fixes: 7fc16da dns/py-dnspython: Update to 2.1.0 for Python 3