These days, the base64 module has 'b64decode', which can tolerate a
str or a bytes as input. Switched to using that, and also, imported it
under a nice short name 'b64'.
In the process, removed the obsolete equivocation between
base64.decodebytes and base64.decodestring. That was there to cope
with Python 2 - but the assert statement right next to it has been
enforcing P3 since commit 2ec2b796ed24cb5 two years ago!
de47ec2f5f