bpo-45459: Add Py_buffer to limited API (GH-29991)

- [x] ``Py_buffer`` struct
- [x] ``PyBuffer_*()`` API functions
- [x] ``PyBUF_*`` constants
- [x] ``Py_bf_getbuffer`` and ``Py_bf_releasebuffer`` type slots
- [x] ``PyMemoryView_FromBuffer()`` API
- [x] tests for limited API
- [x] ``make regen-limited-abi``
- [x] documentation update
- [ ] export ``PyPickleBuffer*()`` API ???

Lib/test/test_stable_abi_ctypes.py

@@ -28,6 +28,14 @@ SYMBOL_NAMES = (
     "PyBaseObject_Type",
     "PyBool_FromLong",
     "PyBool_Type",
+    "PyBuffer_FillContiguousStrides",
+    "PyBuffer_FillInfo",
+    "PyBuffer_FromContiguous",
+    "PyBuffer_GetPointer",
+    "PyBuffer_IsContiguous",
+    "PyBuffer_Release",
+    "PyBuffer_SizeFromFormat",
+    "PyBuffer_ToContiguous",
     "PyByteArrayIter_Type",
     "PyByteArray_AsString",
     "PyByteArray_Concat",
@@ -381,6 +389,7 @@ SYMBOL_NAMES = (
     "PyMemberDescr_Type",
     "PyMember_GetOne",
     "PyMember_SetOne",
+    "PyMemoryView_FromBuffer",
     "PyMemoryView_FromMemory",
     "PyMemoryView_FromObject",
     "PyMemoryView_GetContiguous",
@@ -470,8 +479,10 @@ SYMBOL_NAMES = (
     "PyObject_CallNoArgs",
     "PyObject_CallObject",
     "PyObject_Calloc",
+    "PyObject_CheckBuffer",
     "PyObject_CheckReadBuffer",
     "PyObject_ClearWeakRefs",
+    "PyObject_CopyData",
     "PyObject_DelItem",
     "PyObject_DelItemString",
     "PyObject_Dir",
@@ -489,6 +500,7 @@ SYMBOL_NAMES = (
     "PyObject_GetAIter",
     "PyObject_GetAttr",
     "PyObject_GetAttrString",
+    "PyObject_GetBuffer",
     "PyObject_GetItem",
     "PyObject_GetIter",
     "PyObject_HasAttr",

Lib/test/test_xxlimited.py

@@ -58,6 +58,17 @@ class TestXXLimited(CommonTests, unittest.TestCase):
         with self.assertRaises(self.module.Error):
             raise self.module.Error
 
+    def test_buffer(self):
+        xxo = self.module.Xxo()
+        self.assertEqual(xxo.x_exports, 0)
+        b1 = memoryview(xxo)
+        self.assertEqual(xxo.x_exports, 1)
+        b2 = memoryview(xxo)
+        self.assertEqual(xxo.x_exports, 2)
+        b1[0] = 1
+        self.assertEqual(b1[0], 1)
+        self.assertEqual(b2[0], 1)
+
 
 class TestXXLimited35(CommonTests, unittest.TestCase):
     module = xxlimited_35