Three minor fixes for Python packages on Darwin.

aiohttp and uvloop just need tests disabling; I've tested the resulting packages still work, at least for my use case.

The issue with psutil was correctly diagnosed in #82524, but that hit a stumble with the boolean logic, so this fixes that.

The test disablement in uvloop is slightly unusual, but that's because the way the tests are run is already slightly unusual, and so disabledTests didn't work.

Motivation for this change

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.