Calculate 6 business days
Bug

if self.return_date.isoweekday() is 1 or 7:


Is always true because 7 is true, adding parenthesis will help you understand:

 if  ( self.return_date.isoweekday() is 1 ) or 7:


To get proper behaviour use:

 if self.return_date.isoweekday() in (1, 7):


Repetition

self.return_date = self.return_date + timedelta(2) 
return self.return_date - timedelta(2)


And:

self.return_date = self.return_date + timedelta(1) 
return self.return_date - timedelta(2)


Use a if/ternary for the first timedelta arguments and flatten these lines.

No class

You have a single function with a single argument, simplify and avoid a class. (You can re-use your function without a class, just put it in a separate module and import it.)