Change made automatically with:

    for name in \
        PyObject_Vectorcall \
        Py_TPFLAGS_HAVE_VECTORCALL \
        PyObject_VectorcallMethod \
        PyObject_FastCallDict \
        PyVectorcall_Function \
        PyObject_CallOneArg \
        PyObject_CallMethodNoArgs \
        PyObject_CallMethodOneArg \
    ;
    do
        echo $name
        git grep -lwz _$name | xargs -0 sed -i "s/\b_$name\b/$name/g"
    done

This partially reverts commit 2ff58a2
which added PyObject_CallNoArgs to the 3.9+ stable ABI. This should not
be done; there are enough other call APIs in the stable ABI to choose from.

Mark all newly public functions as added in 3.9.
Add a note about the 3.8 provisional names.
Add notes on public API.