duplicity: use new b2sdk python package

The backblaze library moved into the b2sdk package.
If it’s not used, duplicity fails loading the b2:// backend.

> BackendException: B2 backend requires B2 Python SDK (pip install
  b2sdk)

pkgs/tools/backup/duplicity/default.nix

@@ -46,9 +46,8 @@ pythonPackages.buildPythonApplication rec {
     librsync
   ];
 
-  propagatedBuildInputs = [
-    backblaze-b2
-  ] ++ (with pythonPackages; [
+  propagatedBuildInputs = with pythonPackages; [
+    b2sdk
     boto
     cffi
     cryptography
@@ -65,7 +64,7 @@ pythonPackages.buildPythonApplication rec {
     future
   ] ++ stdenv.lib.optionals (!isPy3k) [
     enum
-  ]);
+  ];
 
   checkInputs = [
     gnupg # Add 'gpg' to PATH.