Make Base64 decoding strict by default

Feature or enhancement

Earlier specifications of Base64 encoding (e.g. RFC 1421, RFC 2045) allowed to ignore any non-alphabetical characters, and that was the behavior of the base64 module. The later specifications (RFC 3548, RFC 4648) require that non-alphabetic characters should be errors, unless opposite is explicitly specified. The Base64 decoder in Python got the option to enable more strict behavior, and now it got option to specify what exactly characters can be ignored.

I think that we should make Base64 decoding strict by default. I am not sure when we should do this, in 3.15 or in future versions. There is no reasonable way to use warnings, they would do more harm.

cc @gpshead

Comienza revisando las opciones actuales de decodificación del módulo base64 y los requisitos de RFC 3548 y RFC 4648 descritos en el issue. Determina las implicaciones para la compatibilidad y la política de releases de cambiar el comportamiento predeterminado, incluido si el cambio debe incorporarse en Python 3.15 o en una versión posterior. Se considera terminado cuando se haya decidido el comportamiento predeterminado y se haya acordado un plan de migración o compatibilidad.