Replace base64_cipher.py with an easy to understand version (#3925)

* rename base64_cipher.py to base64_encoding.py

* edit base64_encoding.py

* import necessary modules inside doctests

* make it behave like the official implementation

* replace format with f-string where possible

* replace format with f-string

Co-authored-by: Christian Clauss <cclauss@me.com>

* fix: syntax error due to closing parenthese

* reformat code

Co-authored-by: Christian Clauss <cclauss@me.com>