Adds check if str(handle) correctly converted the object, and throw py::error_already_set if not.

Similar to pybind#2392, but does not depend on pybind#2409.

Splitting out this PR from pybind#2409 to make that PR as simple as possible.

Net effects of this PR:
* Adds missing test coverage.
* Changes TypeError to UnicodeDecodeError for Python 2.

This PR has two commits. Please do not squash, to make the behavior change obvious in the commit history.

Author: rwgk

include/pybind11/pytypes.h

@@ -920,7 +920,7 @@ class str : public object {
         Return a string representation of the object. This is analogous to
         the ``str()`` function in Python.
     \endrst */
-    explicit str(handle h) : object(raw_str(h.ptr()), stolen_t{}) { }
+    explicit str(handle h) : object(raw_str(h.ptr()), stolen_t{}) { if (!m_ptr) throw error_already_set(); }
 
     operator std::string() const {
         object temp = *this;
@@ -945,8 +945,8 @@ class str : public object {
     /// Return string representation -- always returns a new reference, even if already a str
     static PyObject *raw_str(PyObject *op) {
         PyObject *str_value = PyObject_Str(op);
-        if (!str_value) throw error_already_set();
 #if PY_MAJOR_VERSION < 3
+        if (!str_value) throw error_already_set();
         PyObject *unicode = PyUnicode_FromEncodedObject(str_value, "utf-8", nullptr);
         Py_XDECREF(str_value); str_value = unicode;
 #endif

tests/test_pytypes.py

@@ -114,7 +114,7 @@ def __repr__(self):
     malformed_utf8 = b"\x80"
     assert m.str_from_object(malformed_utf8) is malformed_utf8  # Probably surprising.
     if env.PY2:
-        with pytest.raises(TypeError):
+        with pytest.raises(UnicodeDecodeError):
             m.str_from_handle(malformed_utf8)
     else:
         assert m.str_from_handle(malformed_utf8) == "b'\\x80'"