py::isinstance<py::str>(py_bytes_object) should not return true

[jbarlow83]

Issue description

In pybind 2.2.3, the template specialization py::isinstance<py::str>() returns True when used on py::bytes objects. This is incorrect in both Py2 and 3 (since py::str is unicode in Py2).

py::instance(obj, py::str().get_type()) works correctly.

Reproducible example code

    m.def("test_isinstance",
        []() {
            py::bytes b = "123";
            py::print(py::isinstance<py::str>(b));
        }
    );

>>> pybind11.test_isinstance()
True

[jagerman]

The reason for the failure is that py::str's check function permissively allows both str and bytes objects (or under Python 2: both unicode and str objects). This is mainly here for Python 2 compatibility to allow str objects to be automatically converted to unicode objects.

We can easily add a isinstance<str> specialization does only checks against str (Py 3) or unicode (Py 2) to keep the up-converting behaviour and fix isinstance. That's going to work as expected under Python 3, but the behaviour under Python 2 is going to be a little strange:

m.def("f", [](py::object o) { return py::isinstance<py::str>(o); });

will result in:

m.f("abc")

returning True under Python 3 but False under Python 2, which might be a little surprising.

I don't think it's necessarily wrong for isinstance to work that way; just perhaps surprising.

[bstaletic]

Will be fixed in #2256