[proto_support] Handle unicode filesystem walks.
[proto_support] Handle unicode filesystem walks.

On some filesystem configurations (notably; windows), scandir's os.walk
equivalent will yield directory and file names as unicode objects instead
of str.

TBR=martiniss@chromium.org, nodir@chromium.org, a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org

Bug: 937657
Change-Id: Iadb95d3128fbd5edefb58fe070b7863e96a3c472
Reviewed-on: https://chromium-review.googlesource.com/c/1498125
Commit-Queue: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

luci/recipes-py,luci/recipes-py