sysutils/htop: Restrict to Python 2.x

- restrict htop to build with python 2.7 only.

PR: 194446
Differential Revision:	https://reviews.freebsd.org/D1151
Submitted by: Andrew Berg <aberg010@my.hennepintech.edu>
Approved by: maintainer timeout (2+ weeks)
Approved by: koobs (mentor)
Cleanup plist