python3Packages.urllib3: disable tests again

pkgs/development/python-modules/urllib3/default.nix

@@ -47,13 +47,22 @@ buildPythonPackage rec {
     trustme
   ];
 
-  disabledTests = if stdenv.hostPlatform.isAarch64 then
-    [
-      "test_connection_closed_on_read_timeout_preload_false"
-      "test_ssl_failed_fingerprint_verification"
-      ]
-  else
-    null;
+  # Tests in urllib3 are mostly timeout-based instead of event-based and
+  # are therefore inherently flaky. On your own machine, the tests will
+  # typically build fine, but on a loaded cluster such as Hydra random
+  # timeouts will occur.
+  #
+  # The urllib3 test suite has two different timeouts in their test suite
+  # (see `test/__init__.py`):
+  # - SHORT_TIMEOUT
+  # - LONG_TIMEOUT
+  # When CI is in the env, LONG_TIMEOUT will be significantly increased.
+  # Still, failures can occur and for that reason tests are disabled.
+  doCheck = false;
+
+  preCheck = ''
+    export CI # Increases LONG_TIMEOUT
+  '';
 
   pythonImportsCheck = [ "urllib3" ];