migrant ibus-pinyin from py2 to py3

[Wei Tsui]

ibus-pinyin is already able to work correctly under py3, while py2 is too old and dropped by many system.

from the source code of ibus-pinyin from apt, only one-line patch can make it:

[file]setup/ibus-setup-pinyin.in
-- exec python main.py $@
++ exec python3 main.py $@

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ibus-pinyin 1.5.0-4
ProcVersionSignature: Ubuntu 4.15.0-10.11-generic 4.15.3
Uname: Linux 4.15.0-10-generic x86_64
ApportVersion: 2.20.8-0ubuntu10
Architecture: amd64
CurrentDesktop: MATE
Date: Wed Mar 7 22:39:51 2018
SourcePackage: ibus-pinyin
UpgradeStatus: No upgrade log present (probably fresh install)