python3Packages.uvloop: disable problematic test on aarch64
python3Packages.uvloop: disable problematic test on aarch64

This test case gets stuck on our aarch64 builder since the 0.15.0
upgrade, and so the package has not been in the cache for aarch64,
since the job reliably timed out.

The issue didn't get noticed earlier because the package does in fact
build on some aarch64 machines, like my raspberry pi 4.

Reported upstream at https://github.com/MagicStack/uvloop/issues/412.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs