erp5_core: fix addToDate when removing a month

The way addToDate was working with dates was not good, and creating
confusion when removing 1 month from the last days of a 31-day month, as
the previous day had less days than the current month:

date = DateTime(2023, 5, 31)
print date
print addToDate(date, month=-1)
> 2023/05/31 00:00:00 GMT+2
> 2023/05/01 00:00:00 GMT+2

This was even more confusing in March, with february having only 28
days:

date = DateTime(2023, 3, 31)
print date
print addToDate(date, month=-1)
> 2023/03/31 00:00:00 GMT+2
> 2023/03/03 00:00:00 GMT+2

The new behavior is to, when removing a month, if the new day of the new
month is more than the number of days in month to default to the last
day of the month. For exemple, removing one month from 31/05 becomes
30/04, and from there it will add/remove the days as necessary.

The real issue being that removing a month is ambiguous and can mean
a different thing for different people.

For reference, the reference implementation of timedelta in python
doesn't support adding months:

https://docs.python.org/3/library/datetime.html#datetime.timedelta

I hope my solution will make the more sense in ERP5's context.