make-test-python: disallow aliases

[FRidh]

When importing Nixpkgs within Nixpkgs, we should not consider aliases
to ensure we don't rely on them internally.

There are probably more places that need to be converted.

Triggered by #101369 (comment).

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[FRidh]

I expect evaluation errors.

[SuperSandro2000]

I expect evaluation errors.

Ofborg thinks differently.

[FRidh]

Need to add it to the machine configuration instead.

[FRidh]

@GrahamcOfBorg test plasma5

[jonringer]

@GrahamcOfBorg test installer

[jonringer]

just wanted to run some tests to make sure it wasn't fundamentally broken