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_exceptions.py

@@ -0,0 +1,31 @@
+import pytest
+
+
+def test_custom(msg):
+    from pybind11_tests import (MyException, throws1, throws2, throws3, throws4,
+                                throws_logic_error)
+
+    # Can we catch a MyException?"
+    with pytest.raises(MyException) as excinfo:
+        throws1()
+    assert msg(excinfo.value) == "this error should go to a custom type"
+
+    # Can we translate to standard Python exceptions?
+    with pytest.raises(RuntimeError) as excinfo:
+        throws2()
+    assert msg(excinfo.value) == "this error should go to a standard Python exception"
+
+    # Can we handle unknown exceptions?
+    with pytest.raises(RuntimeError) as excinfo:
+        throws3()
+    assert msg(excinfo.value) == "Caught an unknown exception!"
+
+    # Can we delegate to another handler by rethrowing?
+    with pytest.raises(MyException) as excinfo:
+        throws4()
+    assert msg(excinfo.value) == "this error is rethrown"
+
+    # "Can we fall-through to the default handler?"
+    with pytest.raises(RuntimeError) as excinfo:
+        throws_logic_error()
+    assert msg(excinfo.value) == "this error should fall through to the standard handler"