Clone of @virtuald's PR #2112 with minor enhancements. (#3215)

* Add py::raise_from to enable chaining exceptions on Python 3.3+

* Use 'raise from' in initialization

* Documenting the exact base version of _PyErr_FormatVFromCause, adding back `assert`s.

Co-authored-by: Dustin Spicuzza <dustin@virtualroadside.com>

tests/test_exceptions.py

@@ -24,6 +24,22 @@ def test_error_already_set(msg):
     assert msg(excinfo.value) == "foo"
 
 
+@pytest.mark.skipif("env.PY2")
+def test_raise_from(msg):
+    with pytest.raises(ValueError) as excinfo:
+        m.raise_from()
+    assert msg(excinfo.value) == "outer"
+    assert msg(excinfo.value.__cause__) == "inner"
+
+
+@pytest.mark.skipif("env.PY2")
+def test_raise_from_already_set(msg):
+    with pytest.raises(ValueError) as excinfo:
+        m.raise_from_already_set()
+    assert msg(excinfo.value) == "outer"
+    assert msg(excinfo.value.__cause__) == "inner"
+
+
 def test_cross_module_exceptions(msg):
     with pytest.raises(RuntimeError) as excinfo:
         cm.raise_runtime_error()