bpo-41710: Add _PyTime_AsTimespec_clamp() (GH-28629)

Add the _PyTime_AsTimespec_clamp() function: similar to
_PyTime_AsTimespec(), but clamp to _PyTime_t min/max and don't raise
an exception.

PyThread_acquire_lock_timed() now uses _PyTime_AsTimespec_clamp() to
remove the Py_UNREACHABLE() code path.

* Add _PyTime_AsTime_t() function.
* Add PY_TIME_T_MIN and PY_TIME_T_MAX constants.
* Replace _PyTime_AsTimeval_noraise() with _PyTime_AsTimeval_clamp().
* Add pytime_divide_round_up() function.
* Fix integer overflow in pytime_divide().
* Add pytime_divmod() function.

Modules/selectmodule.c

@@ -344,7 +344,7 @@ select_select_impl(PyObject *module, PyObject *rlist, PyObject *wlist,
                 n = 0;
                 break;
             }
-            _PyTime_AsTimeval_noraise(timeout, &tv, _PyTime_ROUND_CEILING);
+            _PyTime_AsTimeval_clamp(timeout, &tv, _PyTime_ROUND_CEILING);
             /* retry select() with the recomputed timeout */
         }
     } while (1);