net-mgmt/py-snmp4{-apps,-mibs}: Rename to match canonical upstream name

Rename the following ports:

• net-mgmt/py-snmp4 -> net-mgmt/py-pysnmp
• net-mgmt/py-snmp4-apps -> net-mgmt/py-pysnmp-apps
• net-mgmt/py-snmp4-mibs -> net-mgmt/py-pysnmp-mibs

Bump PORTREVISION for net/exaddos + net-mgmt/py-pysmi

PR: 235120
Submitted by: John W. O'Brien <john@saltant.com>
Reviewed by: koobs
Approved by: miwi (mentor), maintainer
Differential Revision: https://reviews.freebsd.org/D19134