Fix checkdigit calculation for ISBN10

[eumiro]

The code calculating checkdigit for ISBN10 runs a loop for numbers between 1 and 10 included. There are two issues:

1. it ignores the possibility of getting the checkdigit of 0 and then raises an UnboundLocalError because the variable valid_check_digit is never set
2. it actually does not have to run a loop to calculate this variable, therefore a minor performance optimization is possible

This PR includes an added test with a valid ISBN10 ending with a 0 that failed earlier but passes now.

[yezz123]

LGTM 🚀