duplicity: S3 backups fail with "boto" not being found.

It seems that duplicity needs boto instead of boto3.
They apparently have different packages.

I decided to keep both,
as there may be another backend depending on it.

pkgs/tools/backup/duplicity/default.nix

@@ -70,6 +70,7 @@ pythonPackages.buildPythonApplication rec {
 
   pythonPath = with pythonPackages; [
     b2sdk
+    boto
     boto3
     cffi
     cryptography