Use python3 for gsutil bootstrapping

R=gavinmak@google.com

Change-Id: I6aaac69c9712e4d639e8804ab603bc18190d88be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3570169Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

ensure_bootstrap

@@ -40,7 +40,7 @@ source "$base_dir/cipd_bin_setup.sh"
 cipd_bin_setup
 
 # Sync "gsutil.py".
-python "$base_dir/gsutil.py" -- version 1> /dev/null &
+python3 "$base_dir/gsutil.py" -- version 1> /dev/null &
 
 # Sync all the pylint versions.
 for script in "$base_dir"/pylint-[0-9].[0-9]; do

gsutil.py.bat

@@ -20,4 +20,4 @@ IF %errorlevel% NEQ 0 (
 set PATH=%PATH%;%~dp0
 
 :: Defer control.
-python "%~dp0gsutil.py" %*
+python3 "%~dp0gsutil.py" %*