Add support for Python 3.13, drop untestable EOL 3.5

[hugovk]

The Python 3.13 release candidate is out now! 🚀

The Release Manager has issued a call to action:

We strongly encourage maintainers of third-party Python projects to prepare their projects for 3.13 compatibilities during this phase, and where necessary publish Python 3.13 wheels on PyPI to be ready for the final release of 3.13.0. Any binary wheels built against Python 3.13.0rc1 will work with future versions of Python 3.13. As always, report any issues to the Python bug tracker.

This PR also drops support for EOL Python 3.5 because it is no longer testable on GitHub Actions: https://github.com/hugovk/idna/actions/runs/10213551799/job/28259160498

I note 3.6-3.7 are also EOL but this project strives for broad compatibility. But lack of CI is a good reason for dropping 3.5 at least.

cycle	release	latest	latest release	support	eol
3.12	2023-10-02	3.12.4	2024-06-06	2025-04-02	2028-10-31
3.11	2022-10-24	3.11.9	2024-04-02	2024-04-01	2027-10-31
3.10	2021-10-04	3.10.14	2024-03-19	2023-04-05	2026-10-31
3.9	2020-10-05	3.9.19	2024-03-19	2022-05-17	2025-10-31
3.8	2019-10-14	3.8.19	2024-03-19	2021-05-03	2024-10-31
3.7	2018-06-26	3.7.17	2023-06-05	2020-06-27	2023-06-27
3.6	2016-12-22	3.6.15	2021-09-03	2018-12-24	2021-12-23
3.5	2015-09-12	3.5.10	2020-09-05	False	2020-09-30