gclient: Allow setting GCLIENT_PY3 to 0 to run on Python 2.

As we update scripts to run Python 3 by default, we want to provide
developers a way to execute scripts on Python 2 if they encounter
any problems.

This change only affects Linux/Mac and not Windows developers.

Bug: 984182
Change-Id: I7c7bd2a45ef50e340f6f2b85e9bc8b63f22d8678
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2079701
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

fetch

@@ -10,7 +10,12 @@ base_dir=$(dirname "$0")
 PATH=$PATH:$base_dir
 
 if [[ $GCLIENT_PY3 == 1 ]]; then
+  # Explicitly run on Python 3
   PYTHONDONTWRITEBYTECODE=1 exec vpython3 "$base_dir/fetch.py" "$@"
+elif [[ $GCLIENT_PY3 == 0 ]]; then
+  # Explicitly run on Python 2
+  PYTHONDONTWRITEBYTECODE=1 exec vpython "$base_dir/fetch.py" "$@"
 else
+  # Run on Python 2 for now, allows default to be flipped.
   PYTHONDONTWRITEBYTECODE=1 exec vpython "$base_dir/fetch.py" "$@"
 fi

gclient

@@ -27,7 +27,12 @@ fi
 PATH=$PATH:$base_dir
 
 if [[ $GCLIENT_PY3 == 1 ]]; then
+  # Explicitly run on Python 3
   PYTHONDONTWRITEBYTECODE=1 exec vpython3 "$base_dir/gclient.py" "$@"
+elif [[ $GCLIENT_PY3 == 0 ]]; then
+  # Explicitly run on Python 2
+  PYTHONDONTWRITEBYTECODE=1 exec vpython "$base_dir/gclient.py" "$@"
 else
-  PYTHONDONTWRITEBYTECODE=1 exec python "$base_dir/gclient.py" "$@"
+  # Run on Python 2 for now, allows default to be flipped.
+  PYTHONDONTWRITEBYTECODE=1 exec vpython "$base_dir/gclient.py" "$@"
 fi