python: bugfix to pyflakes checker
python: bugfix to pyflakes checker

dont refer to g:syntastic_python_checker since - due to a previous
commit - this is not guaranteed to exist any more.

This change should have been done in the aforementioned commit - but I
failed.

wezhang/vim-setup,wezhang/vim-setup,wezhang/vim-setup,wezhang/vim-setup,wezhang/vim-setup,wezhang/vim-setup,nikmartin/dotfiles