Running `fetch v8` with Python 3.5 produces the following error:
TypeError: the JSON object must be str, not 'bytes'

Adding `.decode("utf-8")` makes it compatible with both versions.

Change-Id: Ib0699b61b24f191559c30f1e7ca8d2c919803d03
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2154108Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>