Add devel/viewvc-devel which uses Python 3

The original PR is from Robert William Vesterman but the bulk of this patch
comes from Yasuhito FUTATSUKI <freebsd-bug-report-yf@yf.bsdclub.org>

PR: 247161,247804
Submitted by: Robert William Vesterman <bob@vesterman.com>,Yasuhito FUTATSUKI <freebsd-bug-report-yf@yf.bsdclub.org>
Approved by: maintainer timeout (7 weeks)