[3.13] gh-119661: Add _Py_SINGLETON() include in Argumenet Clinic (#119712) (#119716)

gh-119661: Add _Py_SINGLETON() include in Argumenet Clinic (#119712)

When the _Py_SINGLETON() is used, Argument Clinic now adds an
explicit "pycore_runtime.h" include to get the macro. Previously, the
macro may or may not be included indirectly by another include.

(cherry picked from commit 7ca74a7)

Author: vstinner