fix: provide useful behavior of default py::slice

[dyollb]

Description

Change the behavior of the default constructor of py::slice to be equivalent to vals[::]. I converted a large code base from boost::python to pybind11, and one of the stumbling blocks was this issue (see #5579).

Without this change, calling a[py::slice()] leads to an error, e.g. like this:

______________________________________________________________________________________ test_list_slicing _______________________________________________________________________________________

    def test_list_slicing():
        li = list(range(100))
        assert li[::2] == m.test_list_slicing(li)
>       assert li[::] == m.test_list_slicing_default(li)
E       SystemError: null argument to internal routine

li         = [0, 1, 2, 3, 4, 5, ...]

../../tests/test_pytypes.py:607: SystemError

Suggested changelog entry:

Change the behavior of the default constructor of ``py::slice`` to be equivalent to ``::`` in Python.

[dyollb]

@henryiii I don't quite understand your comments - the code you are commenting on was not changed by my commit. Shall I commit your change suggestions?

[henryiii]

I commented the suggestion. Yes, it was just right next to your changes and might have contributed to your confusion in #5579 about the end point.

I think this looks good now!