Refactor code so it's readable

[Starmania]

Feature description

I was exploring the git when I was reading this file: https://github.com/TheAlgorithms/Python/blob/f32d611689dc72bda67f1c4636ab1599c60d27a4/ciphers/mixed_keyword_cypher.py

The variable name are really sort, and it make difficulties to read... I think it could be cool if someone take the time to describe a little bit more the code...

[tianyizheng02]

The final print statement isn't enclosed in any __main__ statement either, so that should be done too