Luckily, most code is indented 1 to 4 levels at 2 or 4 spaces per indent. This means that typical code width is limited to be anywhere from 78 to 64 characters wide. That's 77 to 63 if you're actually going with 79 characters, which is what PEP8 et al suggest. Much closer to the 55 to 65 characters of the two or three books I just checked.

You're quite right. The ‘accident’ of 80 comes from the ability of a typewriter to space into what were usually margins.