bpo-35134: Split warnings.h and weakrefobject.h (GH-29042)

Split header files to move the non-limited API to Include/cpython/:

* Include/warnings.h => Include/cpython/warnings.h
* Include/weakrefobject.h => Include/cpython/weakrefobject.h

Exclude PyWeakref_GET_OBJECT() from the limited C API. It never
worked since the PyWeakReference structure is opaque in the limited C
API.

Move _PyWarnings_Init() and _PyErr_WarnUnawaitedCoroutine() to the
internal C API.

Include/cpython/warnings.h

@@ -0,0 +1,20 @@
+#ifndef Py_CPYTHON_WARNINGS_H
+#  error "this header file must not be included directly"
+#endif
+
+PyAPI_FUNC(int) PyErr_WarnExplicitObject(
+    PyObject *category,
+    PyObject *message,
+    PyObject *filename,
+    int lineno,
+    PyObject *module,
+    PyObject *registry);
+
+PyAPI_FUNC(int) PyErr_WarnExplicitFormat(
+    PyObject *category,
+    const char *filename, int lineno,
+    const char *module, PyObject *registry,
+    const char *format, ...);
+
+// DEPRECATED: Use PyErr_WarnEx() instead.
+#define PyErr_Warn(category, msg) PyErr_WarnEx(category, msg, 1)

Include/cpython/weakrefobject.h

@@ -0,0 +1,47 @@
+#ifndef Py_CPYTHON_WEAKREFOBJECT_H
+#  error "this header file must not be included directly"
+#endif
+
+/* PyWeakReference is the base struct for the Python ReferenceType, ProxyType,
+ * and CallableProxyType.
+ */
+struct _PyWeakReference {
+    PyObject_HEAD
+
+    /* The object to which this is a weak reference, or Py_None if none.
+     * Note that this is a stealth reference:  wr_object's refcount is
+     * not incremented to reflect this pointer.
+     */
+    PyObject *wr_object;
+
+    /* A callable to invoke when wr_object dies, or NULL if none. */
+    PyObject *wr_callback;
+
+    /* A cache for wr_object's hash code.  As usual for hashes, this is -1
+     * if the hash code isn't known yet.
+     */
+    Py_hash_t hash;
+
+    /* If wr_object is weakly referenced, wr_object has a doubly-linked NULL-
+     * terminated list of weak references to it.  These are the list pointers.
+     * If wr_object goes away, wr_object is set to Py_None, and these pointers
+     * have no meaning then.
+     */
+    PyWeakReference *wr_prev;
+    PyWeakReference *wr_next;
+};
+
+PyAPI_FUNC(Py_ssize_t) _PyWeakref_GetWeakrefCount(PyWeakReference *head);
+
+PyAPI_FUNC(void) _PyWeakref_ClearRef(PyWeakReference *self);
+
+/* Explanation for the Py_REFCNT() check: when a weakref's target is part
+   of a long chain of deallocations which triggers the trashcan mechanism,
+   clearing the weakrefs can be delayed long after the target's refcount
+   has dropped to zero.  In the meantime, code accessing the weakref will
+   be able to "see" the target object even though it is supposed to be
+   unreachable.  See issue #16602. */
+#define PyWeakref_GET_OBJECT(ref)                           \
+    (Py_REFCNT(((PyWeakReference *)(ref))->wr_object) > 0   \
+     ? ((PyWeakReference *)(ref))->wr_object                \
+     : Py_None)