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_sequences_and_iterators.py

@@ -0,0 +1,78 @@
+import pytest
+
+
+def isclose(a, b, rel_tol=1e-05, abs_tol=0.0):
+    """Like to math.isclose() from Python 3.5"""
+    return abs(a - b) <= max(rel_tol * max(abs(a), abs(b)), abs_tol)
+
+
+def allclose(a_list, b_list, rel_tol=1e-05, abs_tol=0.0):
+    return all(isclose(a, b, rel_tol=rel_tol, abs_tol=abs_tol) for a, b in zip(a_list, b_list))
+
+
+def test_sequence():
+    from pybind11_tests import Sequence, ConstructorStats
+
+    cstats = ConstructorStats.get(Sequence)
+
+    s = Sequence(5)
+    assert cstats.values() == ['of size', '5']
+
+    assert "Sequence" in repr(s)
+    assert len(s) == 5
+    assert s[0] == 0 and s[3] == 0
+    assert 12.34 not in s
+    s[0], s[3] = 12.34, 56.78
+    assert 12.34 in s
+    assert isclose(s[0], 12.34) and isclose(s[3], 56.78)
+
+    rev = reversed(s)
+    assert cstats.values() == ['of size', '5']
+
+    rev2 = s[::-1]
+    assert cstats.values() == ['of size', '5']
+
+    expected = [0, 56.78, 0, 0, 12.34]
+    assert allclose(rev, expected)
+    assert allclose(rev2, expected)
+    assert rev == rev2
+
+    rev[0::2] = Sequence([2.0, 2.0, 2.0])
+    assert cstats.values() == ['of size', '3', 'from std::vector']
+
+    assert allclose(rev, [2, 56.78, 2, 0, 2])
+
+    assert cstats.alive() == 3
+    del s
+    assert cstats.alive() == 2
+    del rev
+    assert cstats.alive() == 1
+    del rev2
+    assert cstats.alive() == 0
+
+    assert cstats.values() == []
+    assert cstats.default_constructions == 0
+    assert cstats.copy_constructions == 0
+    assert cstats.move_constructions >= 1
+    assert cstats.copy_assignments == 0
+    assert cstats.move_assignments == 0
+
+
+def test_map_iterator():
+    from pybind11_tests import StringMap
+
+    m = StringMap({'hi': 'bye', 'black': 'white'})
+    assert m['hi'] == 'bye'
+    assert len(m) == 2
+    assert m['black'] == 'white'
+
+    with pytest.raises(KeyError):
+        assert m['orange']
+    m['orange'] = 'banana'
+    assert m['orange'] == 'banana'
+
+    expected = {'hi': 'bye', 'black': 'white', 'orange': 'banana'}
+    for k in m:
+        assert m[k] == expected[k]
+    for k, v in m.items():
+        assert v == expected[k]