env_setup: Tweak Python 3 detection

Change-Id: I73a6ebe5d6a98c7e15086d511ac2d593dc07a2c7
diff --git a/env_setup/env_setup.sh b/env_setup/env_setup.sh
index e9b5859..5e53823 100644
--- a/env_setup/env_setup.sh
+++ b/env_setup/env_setup.sh
@@ -38,7 +38,8 @@
 
 SETUP_SH="$PW_ROOT/env_setup/.setup.sh"
 
-if [ $(which python3 &> /dev/null) ]; then
+# Try to use Python 3 if possible by default, before Python 2.
+if which python3 &> /dev/null; then
   PYTHON=python3
 else
   PYTHON=python