Use linter with pyenv

[sttobia]

As infomation for everyone using the linter while using pyenv, some small fixes are needed because string-bytes path mix-ups. Return paths need to be decoded. So to speak:

• in bin/init_linter_git_hooks the return of the function get_git_repo_root needs to be changed to:

return repo_root.decode()

• in git_hooks.py, the return of the function get_git_repo_root similarly to:

return run_command_in_folder('git rev-parse --show-toplevel',
                               some_folder_in_root_repo).decode()

• similarly, in linter.py, change the return of the function run_command_in_folder to:

return command_output.decode()

Maybe this could be permanently fixed in a future release.