Since 2f25450e11f6 the on_change_product does not always set a unit price for
purchase line. But as the field is required, it must be filled for line
created.
The fiscal years are created centered on the passed date such that if the
current day change during the execution, the fiscal year is still usable.
Also fiscal year can be created for a range of dates such that it is guarantee
that the dates are in the fiscal year.
And finally remove datetime and dateutil import when they are not used.
Closes#9977
Cédric Krier