enable new behavior only for Python 3, adjust new test to work with Python 2

include/pybind11/stl.h

@@ -144,7 +144,11 @@ template <typename Type, typename Value> struct list_caster {
     using value_conv = make_caster<Value>;
 
     bool load(handle src, bool convert) {
-        if (!isinstance<sequence>(src) || (!isinstance<bytes>(src) && isinstance<str>(src)))
+        if (!isinstance<sequence>(src) || (
+#if PY_MAJOR_VERSION >= 3
+            !isinstance<bytes>(src) &&
+#endif
+            isinstance<str>(src)))
             return false;
         auto s = reinterpret_borrow<sequence>(src);
         value.clear();

tests/test_stl.py

@@ -247,7 +247,7 @@ def test_function_with_string_and_vector_string_arg():
 
 def test_bytes_to_vector_uint8_t():
     """Check if a bytes is implicitly converted to std::vector<uint8_t>, issue #1807"""
-    assert m.func_with_vector_uint8_t_arg(b'abc') == 3
+    assert m.func_with_vector_uint8_t_arg([ord(c) for c in b'abc']) == 3
     with pytest.raises(TypeError):
         m.func_with_vector_uint8_t_arg('stringval')