Update tutorial.rst - less way to handle the version field issue

It would be something roughly around https://github.com/pypa/python-packaging-user-guide/pull/60/files#discussion_r13806274

It's not *that* bad if you do it in a strict way (using a separate `version.py` file) but I believe people will try this technique on `__init__.py` believing it's ok to `exec` stuff in general.