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/_xxtestfuzz/fuzzer.c

@@ -104,7 +104,7 @@ static int fuzz_json_loads(const char* data, size_t size) {
     if (input_bytes == NULL) {
         return 0;
     }
-    PyObject* parsed = _PyObject_CallOneArg(json_loads_method, input_bytes);
+    PyObject* parsed = PyObject_CallOneArg(json_loads_method, input_bytes);
     if (parsed == NULL) {
         /* Ignore ValueError as the fuzzer will more than likely
            generate some invalid json and values */
@@ -263,7 +263,7 @@ static int fuzz_sre_match(const char* data, size_t size) {
     PyObject* pattern = compiled_patterns[idx];
     PyObject* match_callable = PyObject_GetAttrString(pattern, "match");
 
-    PyObject* matches = _PyObject_CallOneArg(match_callable, to_match);
+    PyObject* matches = PyObject_CallOneArg(match_callable, to_match);
 
     Py_XDECREF(matches);
     Py_DECREF(match_callable);