Cython: emit `CYTHON_LIMITED_API`

[amcn]

If the user has specified the limited_api keyword arg on an extension_module invocation, then add CYTHON_LIMITED_API to the C and C++ definitions of the target (along with the existing Py_LIMITED_API).

Cython's support for this functionality was added in version 3.0, so we issue a warning if the detected Cython compiler is not at least this version.

Two tests have been added: one based on Cython's limited API test which aims to test that it works in the case of the user's compiler supporting this functionality, and another to test that a warning is issued in the opposite case.