Polybius Square Cipher python implementation

[pyGuru123]

1> Implement a Polybius Square Cipher encryption and decryption method and wrap it inside a class PolybiusCipher
2> save it to polybius_cipher.py
3> put the file inside decrypto/src folder
4> submit a PR.

Sample implementation : https://github.com/pyGuru123/Decrypto/blob/main/decrypto/src/binary_cipher.py
strictly follow above implementation and PEP8 Guidelines