Consider dropping bespoke pep517 backend

### Is your feature request related to a problem?

_No response_

### Describe the solution you'd like

As per https://cython.readthedocs.io/en/latest/src/userguide/source_files_and_compilation.html#basic-setup-py, it's now possible to build cython extensions in setuptools. Would you consider dropping the bespoke backend?

### Describe alternatives you've considered

.

### Additional context

* https://github.com/aio-libs/frozenlist/issues/577
* https://github.com/cython/cython/issues/5949

### Code of Conduct

- [x] I agree to follow the aio-libs Code of Conduct