Make variant optional for from_base64 / to_base64
Make variant optional for from_base64 / to_base64

The library will safely fall back to `URLSAFE_NO_PADDING` if not specified.
dsebastien/DefinitelyTyped,borisyankov/DefinitelyTyped,dsebastien/DefinitelyTyped,markogresak/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,borisyankov/DefinitelyTyped,mcliment/DefinitelyTyped