Remove unused imports (from `$ pyflake3 .`)

[viccuad]

$ pyflake3 . 
./setup.py:1: 'platform' imported but unused
./setup.py:2: 'sys' imported but unused
./neovim_gui/screen.py:9: undefined name 'xrange'
./neovim_gui/cli.py:8: 'IS_PYTHON3' imported but unused