devel/py-configparser: Allow Python 3.x builds

The upstream changelog for 3.5.0 specifies:

a complete rewrite of the backport; now single codebase working on
Python 2.6 - 3.5. To use on Python 3 import from backports import
configparser instead of the built-in version

Previous versions were limited to supporting Python 2.x

Update USES=python and pkg-descr accordingly

While I'm here, enable NO_ARCH

Reported by: net/turses update (r445859)
Approved by: portmgr (blanket)
MFH: 2017Q3