As I was working ongh-120754 I noticed I kept adding more members and copying out individual members from thefstat call, and that it may be simpler / easier to just stash (and invalidate) the whole stat result rather than individul members. This is preparatory work for

1. Avoid callingisatty on open for regular files (ResolvingAvoid calling isatty() for most open() calls #90102)
2. Reduce system calls by making more members available (Helping implementSpeed up open().read() pattern by reducing the number of system calls #120754)

One important note, and why the member is calledstat_atopen is that the values should only be used as guidance / an estimate. With individual members copied out this was also the case. While it's common for a file to not be modified while python code is reading it, other processes could interact with it and code needs to handle that. Two examples of this that I've come across: It is possible to change anfd soisatty result changes (see:gh-90102 andGH-121941) and afd which is opened in blocking mode may have anioctl used on it to change it to non-blocking (see:gh-109523). The general class of bugs here are commonly called time-of-check to time of use (TOCTOU,https://en.wikipedia.org/wiki/Time-of-check_to_time-of-use)

Given how common some specific patterns are (ex.Path().read_bytes()) it is still worthwhile to optimize those (Ex. disabling buffering results in a over 10% speedup in that case,GH-122111). The existing codepaths treated this correctly as far as I can tell.

This PR is a portion ofGH-121593 which is being split up into smaller, hopefully easier to review chunks. Not callingisatty for regular files makes a small but measurable perf improvement for every "open and read whole regular file" python does.