get_toolchain_if_necessary.py would fail on Python 3 when passing a
string to digest.update - this string needs to be encoded to bytes.
This even stops packaging of new toolchains from working.

With this change digest.update get_toolchain_if_necessary.py works with
Python 2 and Python 3.

As a test the current toolchain directory was repackaged - the same hash
was generated.

Change-Id: Ia682d15be42521a35f4df2e14d1f715d9e42d96f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2234586Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>