add lint tool to check python3 shebang

contrib/devtools/lint-python-shebang.sh

@@ -0,0 +1,11 @@
+#!/bin/bash
+# Shebang must use python3 (not python or python2)
+EXIT_CODE=0
+for PYTHON_FILE in $(git ls-files -- "*.py"); do
+    if [[ $(head -c 2 "${PYTHON_FILE}") == "#!" &&
+          $(head -n 1 "${PYTHON_FILE}") != "#!/usr/bin/env python3" ]]; then
+        echo "Missing shebang \"#!/usr/bin/env python3\" in ${PYTHON_FILE} (do not use python or python2)"
+        EXIT_CODE=1
+    fi
+done
+exit ${EXIT_CODE}