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

[jagerman]

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.

Fixes #1461

[bstaletic]

If I got this right, this won't break code that does the following:

std::string GetStringFromPythonObject( pybind11::object o ) {
        // If it is any kind of string like python object just cast it to std::string
        if ( pybind11::isinstance< bytes >( o ) || pybind11::isinstance< str >( o ) ) {
                return o.cast< std::string >();
        }
        // Else, go through pybind11::str()
        return pybind11::str( o ).cast< std::string >();
}

Is that correct?

[jagerman]

It looks like this PR does break Python 2, where py::isinstance<str> is being used to test whether the value is convertible to a string. This is probably at least somewhat historic: we used to have a check() method which isinstance replaced, but the check was intentionally more permissive for strs.

This might be trickier to fix without breaking things.

[jagerman]

If I got this right, this won't break code that does the following:

Your code is fine; the problem is that we have code expecting that isinstance<str>(o) is going to return true when o is a Py2 str. Your code is fine because isinstance<str>(o) || isinstance<bytes>(o) is the proper test.

[wjakob]

How about only enabling the more restrictive check for Python >= 3.x?

[wjakob]

This had some weird conflicts, I ended up applying the patch directly myself.

[wjakob]

Whoops, I missed the fact that this does indeed break the test suite on 2.7. Reverting & reopening.