Add code that will return bytes() from vim functions in Python-3
Do not use vim_funcs['bufnr'] for modified_buffers segment
Make vim_get_func handle rettype argument better

As usual it was not working well in Python-3
Fix overrides in Python-3

With bytes() keys they were not working well since in Python-3 bytes() is not 
comparable with str().
Allow bytes paths in VCS bindings
Run watcher tests with bytes() arguments