message: Fix shebang for protoc-gen-mypy.sh script
diff: -#!/bin/bash
  +#!/usr/bin/env bash
  DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
  # use PYTHON_EXECUTABLE from CMake to get the right python to execute
  "@PYTHON_EXECUTABLE@" -u "$DIR"/protoc-gen-mypy.py
  