Argument Clinic: the Py_buffer converter only accepts None as the default value

[serhiy-storchaka]

Bug report

The Py_buffer converter only accepts None as the default value, even if None is not valid value for Py_buffer. As result, you need to use tricks for optional Py_buffer parameters:

Py_buffer(py_default="b''") = None

or

Py_buffer(py_default="<unrepresentable>") = None

and then check if arg->buf is NULL.

It should support NULL (for "unrepresentable") and arbitrary bytes literals (or at least b'') as default values.