bpo-39245: Switch to public API for Vectorcall (GH-18460)

The bulk of this patch was generated automatically with:

    for name in \
        PyObject_Vectorcall \
        Py_TPFLAGS_HAVE_VECTORCALL \
        PyObject_VectorcallMethod \
        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

    old=_PyObject_FastCallDict
    new=PyObject_VectorcallDict
    git grep -lwz $old | xargs -0 sed -i "s/\b$old\b/$new/g"

and then cleaned up:

- Revert changes to in docs & news
- Revert changes to backcompat defines in headers
- Nudge misaligned comments

Modules/_sre.c

@@ -1081,7 +1081,7 @@ pattern_subx(PatternObject* self, PyObject* ptemplate, PyObject* string,
             match = pattern_new_match(self, &state, 1);
             if (!match)
                 goto error;
-            item = _PyObject_CallOneArg(filter, match);
+            item = PyObject_CallOneArg(filter, match);
             Py_DECREF(match);
             if (!item)
                 goto error;