The python3_bin_reldir.txt path to find the python binary will
contain backslashes instead of forward slashes on windows, so
this tweaks the wrapper in //python-bin/python3 to handle that
and start working under msys.

Bug: 1168365
Change-Id: I0c83f36f48f229276803c2e5d73ec86ecb55ada1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2721367Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>