This PR adds a C API for receiving notifications when sys.settrace() or
sys.setprofile() are called, addressing gh-144690.

Changes

• Add PyUnstable_SetEvalCallback() and PyUnstable_GetEvalCallback() to
  register/retrieve callbacks for trace/profile changes
• Add PyUnstable_EvalEvent enum with TRACE_SET, TRACE_CLEAR,
  PROFILE_SET, PROFILE_CLEAR events
• Callbacks are invoked from _PyEval_SetTrace(), _PyEval_SetTraceAllThreads(), _PyEval_SetProfile(), and _PyEval_SetProfileAllThreads()

API

typedef int (*PyUnstable_EvalCallback)(PyUnstable_EvalEvent event, void *data);
int PyUnstable_SetEvalCallback(PyUnstable_EvalCallback callback, void *data);
PyUnstable_EvalCallback PyUnstable_GetEvalCallback(void **data);