Fix license classifier

[joostlek]

Hey 👋🏻,

I am currently looking into licensing at Home Assistant, and I found that we could not detect the license of this library properly. According to pyproject.toml documentation about the license field:

If you are using a standard, well-known license, it is not necessary to use this field. Instead, you should use one of the classifiers starting with License ::. (As a general rule, it is a good idea to use a standard, well-known license, both to avoid confusion and because some organizations avoid software whose license is unapproved.)

If you could do a release after this PR, that would be awesome :)