vim-beancount: fix broken

Runtime dependency on python3 'beancount' package needed
for autocompletion engine.

Signed-off-by: Sirio Balmelli <sirio@b-ad.ch>

pkgs/misc/vim-plugins/overrides.nix

@@ -556,6 +556,10 @@ self: super: {
     dependencies = with super; [ vim-maktaba ];
   });
 
+  vim-beancount = super.vim-beancount.overrideAttrs(old: {
+    passthru.python3Dependencies = ps: with ps; [ beancount ];
+  });
+
   vim-codefmt = super.vim-codefmt.overrideAttrs(old: {
     dependencies = with super; [ vim-maktaba ];
   });