treewide: Pass self when overriding Python

Otherwise references to the Python interpreter inside the set are wrong, as demonstrated by:
``` nix
with import <nixpkgs> { };
let
  python' = python3.override {
    packageOverrides = final: prev: { requests = prev.requests.overridePythonAttrs(old: { version = "1337";  }); };
  };
in python'.pkgs.python.pkgs.requests
```
which returns the _non_ overriden requests.

And the same with `self`:
```
with import <nixpkgs> { };
let
  python' = python3.override {
    self = python';
    packageOverrides = final: prev: { requests = prev.requests.overridePythonAttrs(old: { version = "1337";  }); };
  };
in python'.pkgs.python.pkgs.requests
```
which returns the overriden requests.

This can manifest itself as file collisions when constructing environments or as subtly incorrect dependency graphs.

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