Reland "gclient: Add a GCLIENT_PY3 env var to allow users to execute gclient using Python 3."

This is a reland of e656e9883179f680292735a6f1fcea9177b2e81f

Original change's description:
> gclient: Add a GCLIENT_PY3 env var to allow users to execute gclient using Python 3.
> 
> Bug: 1008202
> Change-Id: I23dd70b72089a09c2a7cdc74e2a63c135a924ebe
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1825996
> Reviewed-by: Anthony Polito <apolito@google.com>
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
> Auto-Submit: Edward Lesmes <ehmaldonado@chromium.org>

Bug: 1008202
Change-Id: I5181a7f8efb39bdd6adc99db90663cf3cf3cd0bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1828334
Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

gclient

@@ -19,4 +19,12 @@ if [[ "#grep#fetch#cleanup#diff#setdep#" != *"#$1#"* ]]; then
   esac
 fi
 
-PYTHONDONTWRITEBYTECODE=1 exec python "$base_dir/gclient.py" "$@"
+# Ensure that "depot_tools" is somewhere in PATH so this tool can be used
+# standalone, but allow other PATH manipulations to take priority.
+PATH=$PATH:$base_dir
+
+if [[ $GCLIENT_PY3 == 1 ]]; then
+  PYTHONDONTWRITEBYTECODE=1 exec vpython3 "$base_dir/gclient.py" "$@"
+else
+  PYTHONDONTWRITEBYTECODE=1 exec python "$base_dir/gclient.py" "$@"
+fi

gclient.bat

@@ -12,4 +12,9 @@ call "%~dp0update_depot_tools.bat" %*
 set PATH=%PATH%;%~dp0
 
 :: Defer control.
-python "%~dp0gclient.py" %*
+IF "%GCLIENT_PY3%" == "1" (
+  :: TODO(1003139): Use vpython3 once vpython3 works on Windows.
+  python3 "%~dp0gclient.py" %*
+) ELSE (
+  python "%~dp0gclient.py" %*
+)