Rename `codecs._Encoder`, `codecs._Decoder`, `.encode` and `.decode` `input` argument

`input` argument of `.encode` and `.decode` methods shadows built-in function `input`, which might trigger linter checks and make code editor highlight this variable as built-in.

This shows that nobody is calling these methods using `input` keyword argument:
https://grep.app/search?q=decode%28input%3D&filter[lang][0]=Python
https://grep.app/search?q=encode%28input%3D&filter[lang][0]=Python