gh-69142: add %:z strftime format code

Doc/library/datetime.rst

@@ -2390,7 +2390,6 @@ requires, and these work on all platforms with a standard C implementation.
 |           | ``±HH:MM[:SS[.ffffff]]``       | -04:00, +10:30,        |       |
 |           | (empty string if the object is | +06:34:15,             |       |
 |           | naive).                        | -03:07:12.345216       |       |
-|           | .. versionadded:: 3.12         |                        |       |
 +-----------+--------------------------------+------------------------+-------+
 | ``%Z``    | Time zone name (empty string   | (empty), UTC, GMT      | \(6)  |
 |           | if the object is naive).       |                        |       |

Misc/NEWS.d/next/Library/2022-08-14-18-59-54.gh-issue-69142.6is5Pq.rst

@@ -1 +1 @@
-add %:z strftime format code (generates tzoffset with colons as separator)
+Add ``%:z`` strftime format code (generates tzoffset with colons as separator), see :ref:`strftime-strptime-behavior`.

Modules/_datetimemodule.c

@@ -1511,25 +1511,20 @@ make_somezreplacement(PyObject *object, char *sep, PyObject *tzinfoarg)
 {
     char buf[100];
     PyObject *tzinfo = get_tzinfo_member(object);
-    PyObject *replacement = PyBytes_FromStringAndSize(NULL, 0);
-
-    if (replacement == NULL)
-        return NULL;
-    if (tzinfo == Py_None || tzinfo == NULL)
-        return replacement;
-
-    Py_DECREF(replacement);
 
+    if (tzinfo == Py_None || tzinfo == NULL) {
+        return PyBytes_FromStringAndSize(NULL, 0);
+    }
+
     assert(tzinfoarg != NULL);
     if (format_utcoffset(buf,
                          sizeof(buf),
                          sep,
                          tzinfo,
                          tzinfoarg) < 0)
         return NULL;
-
-    replacement = PyBytes_FromStringAndSize(buf, strlen(buf));
-    return replacement;
+
+    return PyBytes_FromStringAndSize(buf, strlen(buf));
 }
 
 static PyObject *