Fix not allowing program times on event series (API/copy) (#5595)

* Fix not allowing program times on event series (API/copy)

* Return 400 when reading endpoint in event series

* add docs program times not available on event series

* fix isort

doc/api/resources/item_program_times.rst

@@ -5,6 +5,7 @@ Resource description
 --------------------
 
 Program times for products (items) that can be set in addition to event times, e.g. to display seperate schedules within an event.
+Note that ``program_times`` are not available for items inside event series.
 The program times resource contains the following public fields:
 
 .. rst-class:: rest-resource-table

doc/api/resources/items.rst

@@ -142,6 +142,7 @@ variations                              list of objects            A list with o
 program_times                           list of objects            A list with one object for each program time of this item.
                                                                    Can be empty. Only writable during creation,
                                                                    use separate endpoint to modify this later.
+                                                                   Not available for items in event series.
 ├ id                                    integer                    Internal ID of the variation
 ├ value                                 multi-lingual string       The "name" of the variation
 ├ default_price                         money (string)             The price set directly for this variation or ``null``
@@ -243,6 +244,8 @@ Also note that ``variations``, ``bundles``, ``addons`` and  ``program_times`` ar
 bundles, add-ons and program times please use the dedicated nested endpoints. By design this endpoint does not support ``PATCH`` and ``PUT``
 with nested ``variations``, ``bundles``, ``addons`` and/or ``program_times``.
 
+``program_times`` is not available to items in event series.
+
 Endpoints
 ---------