Add Python 3.12 support

[Dmytro Hrankin]

Please add support for Python 3.12. As I see, the library is not tested for Python 3.12, so I am not sure if the library will work correctly: https://github.com/andrew-d/python-multipart/blob/d3d16dae4b061c34fe9d3c9081d9800c49fc1f7a/.github/workflows/test.yaml#L17