Bug#1124393: ibus must not Build-Depends: python-is-python3

[Changwoo Ryu]

control: forwarded -1 https://github.com/ibus/ibus/issues/2844
control: tag -1 + upstream

This is against PEP 394 and should be upstreamed.

engine/test-gnome.py is the only file which causes FTBFS . Other files
don't need changes.