maths/sum_of_digits.py: Streamline benchmarks (#7914)

* maths/sum_of_digits.py: Streamline benchmarks

```
sum_of_digits(262144): 19 -- 0.3128329170285724 seconds
sum_of_digits_recursion(262144): 19 -- 0.34008108399575576 seconds
sum_of_digits_compact(262144): 19 -- 0.6086010000435635 seconds

sum_of_digits(1125899906842624): 76 -- 0.8079068749793805 seconds
sum_of_digits_recursion(1125899906842624): 76 -- 0.8435653329943307 seconds
sum_of_digits_compact(1125899906842624): 76 -- 1.247976207989268 seconds

sum_of_digits(1267650600228229401496703205376): 115 -- 1.6441589999594726 seconds
sum_of_digits_recursion(1267650600228229401496703205376): 115 -- 1.713684624992311 seconds
sum_of_digits_compact(1267650600228229401496703205376): 115 -- 2.2197747920290567 seconds
```

* updating DIRECTORY.md

* Update sum_of_digits.py

* Update sum_of_digits.py

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