Port tests to pytest

Use simple asserts and pytest's powerful introspection to make testing
simpler. This merges the old .py/.ref file pairs into simple .py files
where the expected values are right next to the code being tested.

This commit does not touch the C++ part of the code and replicates the
Python tests exactly like the old .ref-file-based approach.

tests/test_buffers.py

@@ -0,0 +1,57 @@
+import pytest
+from pybind11_tests import Matrix, ConstructorStats
+
+with pytest.suppress(ImportError):
+    import numpy as np
+
+
+@pytest.requires_numpy
+def test_to_python():
+    m = Matrix(5, 5)
+
+    assert m[2, 3] == 0
+    m[2, 3] = 4
+    assert m[2, 3] == 4
+
+    m2 = np.array(m, copy=False)
+    assert m2.shape == (5, 5)
+    assert abs(m2).sum() == 4
+    assert m2[2, 3] == 4
+    m2[2, 3] = 5
+    assert m2[2, 3] == 5
+
+    cstats = ConstructorStats.get(Matrix)
+    assert cstats.alive() == 1
+    del m
+    assert cstats.alive() == 1
+    del m2  # holds an m reference
+    assert cstats.alive() == 0
+    assert cstats.values() == ["5x5 matrix"]
+    assert cstats.copy_constructions == 0
+    # assert cstats.move_constructions >= 0  # Don't invoke any
+    assert cstats.copy_assignments == 0
+    assert cstats.move_assignments == 0
+
+
+@pytest.requires_numpy
+def test_from_python():
+    with pytest.raises(RuntimeError) as excinfo:
+        Matrix(np.array([1, 2, 3]))  # trying to assign a 1D array
+    assert str(excinfo.value) == "Incompatible buffer format!"
+
+    m3 = np.array([[1, 2, 3], [4, 5, 6]]).astype(np.float32)
+    m4 = Matrix(m3)
+
+    for i in range(m4.rows()):
+        for j in range(m4.cols()):
+            assert m3[i, j] == m4[i, j]
+
+    cstats = ConstructorStats.get(Matrix)
+    assert cstats.alive() == 1
+    del m3, m4
+    assert cstats.alive() == 0
+    assert cstats.values() == ["2x3 matrix"]
+    assert cstats.copy_constructions == 0
+    # assert cstats.move_constructions >= 0  # Don't invoke any
+    assert cstats.copy_assignments == 0
+    assert cstats.move_assignments == 0