Cleanup: go through `bootstrap.py` and make sure we specify explicit `open()` encodings

[jieyouxu]

Remark: we should go through bootstrap.py and make sure we specific explicit open() encodings, as far as I am aware there is a reasonable desire to want to make a future change in Python 3.* where open() defaults to UTF-8 encoding, which could break open() on binary files called elsewhere in this script.

Originally posted by @jieyouxu in #152485 (comment)

[ozankenangungor]

I can take this. I will make the change mechanical and keep binary-mode opens unchanged.

@rustbot claim

[Kobzol]

@jieyouxu Do you have some link to the Python change that you describe?

[jieyouxu]

@jieyouxu Do you have some link to the Python change that you describe?

Yes. https://docs.python.org/3.15/whatsnew/3.15.html#whatsnew315-utf8-default. So actually, this issue is obsolete if users are using >= Python 3.15. Didn't occur to me Python 3.15 is already out as a pre-release.

I think my description wasn't particularly accurate, the behavior only is different if the open() uses "r" not "rb".