devel/py-apptools: Enable Python 3.x builds

• Support for Python 3 was introduced with 4.4.0
• Pet portlint (reduce the overall usage of lines in the pkg-descr and separate the USES block).
• No bump of PORTREVISION due style changes only.

PR: 238435
Submitted by: vladimir.chukharev@gmail.com (maintainer)
MFH: 2019Q2