PEP 553 built-in breakpoint() function (bpo-31353) (#3355)

Implement PEP 553, built-in breakpoint() with support from sys.breakpointhook(), along with documentation and tests.  Closes bpo-31353

Python/bltinmodule.c

@@ -422,6 +422,28 @@ builtin_callable(PyObject *module, PyObject *obj)
     return PyBool_FromLong((long)PyCallable_Check(obj));
 }
 
+static PyObject *
+builtin_breakpoint(PyObject *self, PyObject **args, Py_ssize_t nargs, PyObject *keywords)
+{
+    PyObject *hook = PySys_GetObject("breakpointhook");
+
+    if (hook == NULL) {
+        PyErr_SetString(PyExc_RuntimeError, "lost sys.breakpointhook");
+        return NULL;
+    }
+    Py_INCREF(hook);
+    PyObject *retval = _PyObject_FastCallKeywords(hook, args, nargs, keywords);
+    Py_DECREF(hook);
+    return retval;
+}
+
+PyDoc_STRVAR(breakpoint_doc,
+"breakpoint(*args, **kws)\n\
+\n\
+Call sys.breakpointhook(*args, **kws).  sys.breakpointhook() must accept\n\
+whatever arguments are passed.\n\
+\n\
+By default, this drops you into the pdb debugger.");
 
 typedef struct {
     PyObject_HEAD
@@ -2627,6 +2649,7 @@ static PyMethodDef builtin_methods[] = {
     BUILTIN_ANY_METHODDEF
     BUILTIN_ASCII_METHODDEF
     BUILTIN_BIN_METHODDEF
+    {"breakpoint",      (PyCFunction)builtin_breakpoint, METH_FASTCALL | METH_KEYWORDS, breakpoint_doc},
     BUILTIN_CALLABLE_METHODDEF
     BUILTIN_CHR_METHODDEF
     BUILTIN_COMPILE_METHODDEF