There are ways to harden and/or reduce privileges, but shells/scripting languages will always have this issue on any modern OS.
The UNIX way to help prevent that is really to run processes as another user, but people seem to refuse to do so. You should always expect any process running as your UID to be able to access any data owned or visible to your UID.
While it is possible to reduce the risk of disclosure, they are all wack-a-mole preventions protecting the low hanging fruit, not absolute guarantees.
That is purely due to how UNIX works [0]
[0] https://man7.org/linux/man-pages/man7/credentials.7.html
The very idea that you offer a (python) package installer that is gonna pull a tree of code published and updated by random people in an unvetted manner open the door to all the supply chain attacks we are seeing.
Around the same time (early 90s) Java was designed with high isolation in mind but the goal and vision was very different. And Java had its own problems.
I'm saying that because at some point the security problem is gonna really hurt the python ecosystem.