add %:z strftime format code

datetime.isoformat generates the tzoffset with colons, but there
was no format code to make strftime output the same format.

for simplicity and consistency the %:z formatting behaves mostly
as %z, with the exception of adding colons. this includes the
dynamic behaviour of adding seconds and microseconds only when
needed (when not 0).

this fixes the still open "generate" part of this issue:

#69142

Author: ThomasWaldmann

Doc/library/datetime.rst

@@ -2386,6 +2386,12 @@ requires, and these work on all platforms with a standard C implementation.
 |           | string if the object is        | +063415,               |       |
 |           | naive).                        | -030712.345216         |       |
 +-----------+--------------------------------+------------------------+-------+
+| ``%:z``   | UTC offset in the form         | (empty), +00:00,       |       |
+|           | ``±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).       |                        |       |
 +-----------+--------------------------------+------------------------+-------+

Modules/_datetimemodule.c

@@ -1631,7 +1631,7 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
             ntoappend = 1;
         }
         /* A % has been seen and ch is the character after it. */
-        else if (ch == 'z') {
+        else if (ch == 'z' || (ch == ':' && *pin == 'z')) {
             if (zreplacement == NULL) {
                 /* format utcoffset */
                 char buf[100];
@@ -1640,9 +1640,17 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
                 if (zreplacement == NULL) goto Done;
                 if (tzinfo != Py_None && tzinfo != NULL) {
                     assert(tzinfoarg != NULL);
+                    /* %z -> +HHMM, %:z -> +HH:MM */
+                    char *sep;
+                    if (ch == ':') {
+                        sep = ":";
+                        pin++;
+                    } else {
+                        sep = "";
+                    }
                     if (format_utcoffset(buf,
                                          sizeof(buf),
-                                         "",
+                                         sep,
                                          tzinfo,
                                          tzinfoarg) < 0)
                         goto Done;
@@ -1686,7 +1694,7 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
             ntoappend = PyBytes_GET_SIZE(freplacement);
         }
         else {
-            /* percent followed by neither z nor Z */
+            /* percent followed by something else */
             ptoappend = pin - 2;
             ntoappend = 2;
         }