I've merged Idan's PR adding a strict_mode parameter to a2b_base64.  It defaults to False for backwards compatibility.

From a security perspective, it'd be _ideal_ if this were True.  But I expect doing that would break a bunch of existing code and tests that has been relying on some of the former leniency behaviors so I recommended the conservative approach of the old-behavior default.  It'd be a good thing to change it to True, but disruptive.  We need motivating reason to do that.

As it is a new feature due to the new parameter, this is for 3.11.

Workaround for Pythons without this: do a validity check before calling a2b_base64.  I suspect a regex could be constructed for that if you're careful.  If you come up with one, please share it here.