luhn_ex
                                
                                
                                
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                        add ability to calculate or append check digits
FWIW, while I did this I enabled doctests. I noticed that you've got an existing doctest which isn't passing -- 0x1580BB2EA8875 gets treated as the same as 378282246310005, but appears to otherwise be an example of base 16 use.
power_assert is also causing deprecation warnings & I left that alone too.