[fetch] use py3 for fetch

Bug: 939847
Change-Id: I8c73328363a9ffe506db6ed4e46c72ccad238c3a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1845499
Commit-Queue: Anthony Polito <apolito@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>

fetch

@@ -9,4 +9,8 @@ base_dir=$(dirname "$0")
 # standalone, but allow other PATH manipulations to take priority.
 PATH=$PATH:$base_dir
 
-PYTHONDONTWRITEBYTECODE=1 exec vpython "$base_dir/fetch.py" "$@"
+if [[ $GCLIENT_PY3 == 1 ]]; then
+  PYTHONDONTWRITEBYTECODE=1 exec vpython3 "$base_dir/fetch.py" "$@"
+else
+  PYTHONDONTWRITEBYTECODE=1 exec vpython "$base_dir/fetch.py" "$@"
+fi

fetch.bat

@@ -12,4 +12,8 @@ call "%~dp0\update_depot_tools.bat"
 set PATH=%PATH%;%~dp0
 
 :: Defer control.
-vpython "%~dp0\fetch.py" %*
+IF "%GCLIENT_PY3%" == "1" (
+  vpython3 "%~dp0\fetch.py" %*y
+) ELSE (
+  vpython "%~dp0\fetch.py" %*
+)