[issue28977] Document PyObject_CallFunction() special case more explicitly

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

"O" is not the only special case. "S" and "N" unpack a tuple argument too. "O&" converter should return a tuple, otherwise it is an error. All other format codes are illegal for single argument.

I would just deprecate this feature (in PyObject_CallFunction, not in Py_BuildValue). The behavior of PyObject_CallFunction with a single argument can be made more consistent and useful.

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<http://bugs.python.org/issue28977>
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