Switch on "new" buffer API.

"Old" buffer API will removed in Python 3.10. This is also fix #7930.

python/google/protobuf/pyext/message.cc

@@ -1927,9 +1927,8 @@ PyObject* SetAllowOversizeProtos(PyObject* m, PyObject* arg) {
 }
 
 static PyObject* MergeFromString(CMessage* self, PyObject* arg) {
-  const void* data;
-  Py_ssize_t data_length;
-  if (PyObject_AsReadBuffer(arg, &data, &data_length) < 0) {
+  Py_buffer data;
+  if (PyObject_GetBuffer(arg, &data, PyBUF_SIMPLE) < 0) {
     return NULL;
   }
 
@@ -1942,7 +1941,8 @@ static PyObject* MergeFromString(CMessage* self, PyObject* arg) {
   const char* ptr;
   internal::ParseContext ctx(
       depth, false, &ptr,
-      StringPiece(static_cast<const char*>(data), data_length));
+      StringPiece(static_cast<const char*>(data.buf), data.len));
+  PyBuffer_Release(&data);
   ctx.data().pool = factory->pool->pool;
   ctx.data().factory = factory->message_factory;
 
@@ -1968,9 +1968,9 @@ static PyObject* MergeFromString(CMessage* self, PyObject* arg) {
     // TODO(jieluo): Raise error and return NULL instead.
     // b/27494216
     PyErr_Warn(nullptr, "Unexpected end-group tag: Not all data was converted");
-    return PyInt_FromLong(data_length - ctx.BytesUntilLimit(ptr));
+    return PyInt_FromLong(data.len - ctx.BytesUntilLimit(ptr));
   }
-  return PyInt_FromLong(data_length);
+  return PyInt_FromLong(data.len);
 }
 
 static PyObject* ParseFromString(CMessage* self, PyObject* arg) {

python/tox.ini

@@ -14,10 +14,7 @@ setenv =
 commands =
     python setup.py -q build_py
     python: python setup.py -q build
-    # --warnings_as_errors disabled until we update the Python C extension. See:
-    # https://github.com/protocolbuffers/protobuf/issues/7930
-    # cpp: python setup.py -q build --cpp_implementation --warnings_as_errors --compile_static_extension
-    cpp: python setup.py -q build --cpp_implementation --compile_static_extension
+    cpp: python setup.py -q build --cpp_implementation --warnings_as_errors --compile_static_extension
     python: python setup.py -q test -q
     cpp: python setup.py -q test -q --cpp_implementation
     python: python setup.py -q test_conformance