pythonPackages.boto3: disable tests (fails due to network access)

The test failure started happening with commit
8d18f67a97 ("awscli: 1.11.45 -> 1.11.75"). That commit
also updates botocore, a dependency of boto3.

This unbreaks 'nixops' (a dependee).

pkgs/top-level/python-packages.nix

@@ -3187,6 +3187,9 @@ in {
       runHook postCheck
     '';
 
+    # Network access
+    doCheck = false;
+
     meta = {
       homepage = https://github.com/boto/boto3;
       license = stdenv.lib.licenses.asl20;