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/_sqlite/cache.c

@@ -183,7 +183,7 @@ PyObject* pysqlite_cache_get(pysqlite_Cache* self, PyObject* key)
             }
         }
 
-        /* We cannot replace this by _PyObject_CallOneArg() since
+        /* We cannot replace this by PyObject_CallOneArg() since
          * PyObject_CallFunction() has a special case when using a
          * single tuple as argument. */
         data = PyObject_CallFunction(self->factory, "O", key);