Fix buffer protocol inheritance

Fixes #878.

tests/test_buffers.py

@@ -36,6 +36,7 @@ def test_from_python():
 @pytest.unsupported_on_pypy
 def test_to_python():
     m = Matrix(5, 5)
+    assert memoryview(m).shape == (5, 5)
 
     assert m[2, 3] == 0
     m[2, 3] = 4
@@ -63,6 +64,16 @@ def test_to_python():
     assert cstats.move_assignments == 0
 
 
+@pytest.unsupported_on_pypy
+def test_inherited_protocol():
+    """SquareMatrix is derived from Matrix and inherits the buffer protocol"""
+    from pybind11_tests import SquareMatrix
+
+    matrix = SquareMatrix(5)
+    assert memoryview(matrix).shape == (5, 5)
+    assert np.asarray(matrix).shape == (5, 5)
+
+
 @pytest.unsupported_on_pypy
 def test_ptmf():
     for cls in [PTMFBuffer, ConstPTMFBuffer, DerivedPTMFBuffer]: