> If the input to b64decode is a str, just do a encode('ascii')
> operation on it and proceed.  If that fails, it wasn't valid 
> Base64 to begin with.

On unicode encode error, should we raise an UnicodeEncodeError or a 
binascii.Error?

And there is also the problem of base64.b64decode() 
alternate "characters". Should we accept non-ASCII alternate 
characters?
   base64.b64decode('01a\xfeb\xffcd', altchars=b'\xfe\xff')

For the example, the result depends on the choosen charset:
 - ASCII (strict): encode input text raise an UnicodeDecodeError
 - ISO-8859-1 (ignore): works as expected
 - UTF-8 (strict): unexpected result

The only valid choice is ASCII because ISO-8859-1 or UTF-8 will 
reintroduce bytes/character mixture.