workdays

Extend python datetime with excel-like workday addition/subtraction functionality:

NETWORKDAYS(start_date,end_date,holidays)

Returns the number of whole working days between start_date and
end_date (inclusive of both start_date and end_date). Working days
exclude weekends and any dates identified in holidays. Use NETWORKDAYS
to calculate employee benefits that accrue based on the number of
days worked during a specific term.

WORKDAY(start_date,days,[holidays])

Returns a number that represents a date that is the indicated number
of working days before or after a date (the starting date). Working
days exclude weekends and any dates identified as holidays. Use
WORKDAY to exclude weekends or holidays when you calculate invoice
due dates, expected delivery times, or the number of days of work
performed.

This module has similarities with the BusinessHours module - you may
want to check it out as well to see which one better fits your needs.