gh-139156: Use PyBytesWriter in the UTF-7 encoder (#139248)

Replace PyBytes_FromStringAndSize() and _PyBytes_Resize() with the
PyBytesWriter API.

Author: vstinner

Objects/unicodeobject.c

@@ -4897,33 +4897,27 @@ _PyUnicode_EncodeUTF7(PyObject *str,
                       int base64WhiteSpace,
                       const char *errors)
 {
-    int kind;
-    const void *data;
-    Py_ssize_t len;
-    PyObject *v;
-    int inShift = 0;
-    Py_ssize_t i;
-    unsigned int base64bits = 0;
-    unsigned long base64buffer = 0;
-    char * out;
-    const char * start;
-
-    kind = PyUnicode_KIND(str);
-    data = PyUnicode_DATA(str);
-    len = PyUnicode_GET_LENGTH(str);
-
-    if (len == 0)
+    Py_ssize_t len = PyUnicode_GET_LENGTH(str);
+    if (len == 0) {
         return Py_GetConstant(Py_CONSTANT_EMPTY_BYTES);
+    }
+    int kind = PyUnicode_KIND(str);
+    const void *data = PyUnicode_DATA(str);
 
     /* It might be possible to tighten this worst case */
-    if (len > PY_SSIZE_T_MAX / 8)
+    if (len > PY_SSIZE_T_MAX / 8) {
         return PyErr_NoMemory();
-    v = PyBytes_FromStringAndSize(NULL, len * 8);
-    if (v == NULL)
+    }
+    PyBytesWriter *writer = PyBytesWriter_Create(len * 8);
+    if (writer == NULL) {
         return NULL;
+    }
 
-    start = out = PyBytes_AS_STRING(v);
-    for (i = 0; i < len; ++i) {
+    int inShift = 0;
+    unsigned int base64bits = 0;
+    unsigned long base64buffer = 0;
+    char *out = PyBytesWriter_GetData(writer);
+    for (Py_ssize_t i = 0; i < len; ++i) {
         Py_UCS4 ch = PyUnicode_READ(kind, data, i);
 
         if (inShift) {
@@ -4986,9 +4980,7 @@ _PyUnicode_EncodeUTF7(PyObject *str,
         *out++= TO_BASE64(base64buffer << (6-base64bits) );
     if (inShift)
         *out++ = '-';
-    if (_PyBytes_Resize(&v, out - start) < 0)
-        return NULL;
-    return v;
+    return PyBytesWriter_FinishWithPointer(writer, out);
 }
 
 #undef IS_BASE64