Make py::isinstance<py::str> less permissive

py::str's uses a permissive check function that allows both str (Py 3)/
unicode (Py 2), *and* bytes (Py 3)/str (Py 2) so that a `py::str` can
be created from either, but this makes the default `isinstance`
implementation overly permissive.

Author: jagerman

include/pybind11/pytypes.h

@@ -895,6 +895,9 @@ class str : public object {
 };
 /// @} pytypes
 
+// str::check_ is too permissive for `isinstance` (in particular it allows a `bytes` object)
+template <> inline bool isinstance<str>(handle obj) { return PyUnicode_Check(obj.ptr()); }
+
 inline namespace literals {
 /** \rst
     String literal version of `str`

tests/test_pytypes.cpp

@@ -269,4 +269,7 @@ TEST_SUBMODULE(pytypes, m) {
     m.def("print_failure", []() { py::print(42, UnregisteredType()); });
 
     m.def("hash_function", [](py::object obj) { return py::hash(obj); });
+
+    // test_str_isinstance
+    m.def("is_str_instance", [](py::object o) { return py::isinstance<py::str>(o); });
 }

tests/test_pytypes.py

@@ -238,3 +238,8 @@ class Unhashable(object):
     assert m.hash_function(Hashable(42)) == 42
     with pytest.raises(TypeError):
         m.hash_function(Unhashable())
+
+
+def test_str_isinstance():
+    assert m.is_str_instance(u"abc")
+    assert not m.is_str_instance(b"abc")