Add value factory parameter to `dict.fromkeys` to enable use with mutable values

[Stefan2 (Stefan Pochmann)]

I mean map’s implementation would already naturally do what I want it to if it weren’t artificially forbidden to do it. We talked about it here:

Don't forbid `map(nullary_func)`

map(f, *iterables) calls f with elements from the iterables until one of them runs out of elements. Except when there are zero iterables. That’s forbidden, for no apparent reason. With the above general rule, map(f) would give an iterator infinitely calling f with zero arguments. Infinite because none of the zero iterables ever run out of elements. So for example map(random) would be an infinite iterator of random numbers, map(time) would provide timestamps, and map(count) would provide count…