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
Allow bytes paths in watchers
Run watcher tests with bytes() arguments
Replace os.path.walk with os.walk

os.path.walk was removed in python-3
Allow powerline-lint and sphinx to run with fake vim object