Without a license, other people are just allowed to look at it, and click the “fork” button on GitHub. Because the default license is “all rights reserved”.
So if you do want other people to be able to use, modify, and share this code, please do add an open source license. For permissive licenses I recommend the Apache 2.0 License because it is legally precise, but the various MIT license variants are fine as well. But only add a open source license if you feel comfortable with the rights you're giving to the public. Some authors aren't comfortable with commercial use, in which case open source licenses are not appropriate.
In order for GitHub to detect the license, put the license text into a file called
LICENSE (or named similarly). It would also be helpful to add a copyright header to the script itself, and to note the license in your README. If you want to publish your script to PyPI, you can add license metadata to the