Revert "python3Packages.beancount: 2.3.6 -> 3.0.0"

This reverts commit 72e7936, which
upgrades beancount to 3.0.0. Beancount v3 is an unstable version with
major changes compared to the stable v2 version. Tools including fava
have not supported it yet, and current v3 build in nixpkgs is broken
(failed to run).

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

@@ -18,15 +18,15 @@
 }:
 
 buildPythonPackage rec {
-  version = "3.0.0";
+  version = "2.3.6";
   format = "setuptools";
   pname = "beancount";
 
   disabled = !isPy3k;
 
   src = fetchPypi {
     inherit pname version;
-    hash = "sha256-z2aGhpx+o+78CU7hPthmv196K7DGHk1PXfPjX4Rs/98=";
+    hash = "sha256-gB+Tvta1fS4iQ2aIxInVob8fduIQ887RhoB1fmDTR1o=";
   };
 
   # Tests require files not included in the PyPI archive.