remove calls to utcnow and utcfromtimestamp

[graingert]

#3288 fixed most of these but some remained

[bdarnell]

datetime.datetime.fromtimestamp(x, datetime.timezone.utc).replace(tzinfo=None) is a non-deprecated way to do what we were doing before, but it goes against the spirit of the deprecation. In #3288 I fixed this by using aware datetimes in most cases instead of trying to continue to use naive ones. I think we want to do the same here (and get rid of all the replace(tzinfo=None) calls). But I know this is untested code (or it would have been caught already), so I'm a little wary of touching it. Did you find that these replace() calls were necessary or were you just trying to preserve the original semantics?