[FIX] base: traceback in `read_group`

An issue originally occurs in the Event module.
- Go to Events >> Reports >> Events
- Add a "Group By" in the search view: "Event Month"
- A mighty traceback pops up

This is because the rows and the columns are grouped by the same field
(`event_date:month`).

In method `_read_group_format_result`, a group_by of type "date" or
"datetime" is replaced by a "(range_start/range_end, label)" tuple. The
first element of the loop on `annotated_groupbys` is handled correctly,
but the next identical element will crash on `value + gb['interval']`,
since `value` is now a tuple and not a date/datetime anymore.

opw-686242