Add doctest to maths/sieve_of_eratosthenes.py and remove other/finding_primes.py (#1078)

Both of the two files implemented sieve of eratosthenes.
However, there was a bug in other/finding_primes.py, and the time complexity was larger than the other.
Therefore, remove other/finding_primes.py and add doctest tomaths/sieve_of_eratosthenes.py.