buku: fix build

Failing Hydra build: https://hydra.nixos.org/build/148285785

The underlying issue is `flask-admin` not working with newer versions of
`SQLAlchemy` and `SQLAlchemy-Utils`[1].

I decided to only fix `buku` now since this is the package I wanted to
take care of initially. Also, multiple versions of python packages - a
measure that would be necessary for `flask-admin` - are generally
discouraged[2].

[1] 1.5.8 works with `SQLAlchemy`, but is broken with `SQLAlchemy-Utils`
    at 0.37, see also pallets-eco/flask-admin#2113
[2] NixOS#127707

pkgs/applications/misc/buku/default.nix

@@ -1,6 +1,26 @@
 { lib, python3, fetchFromGitHub }:
 
-with python3.pkgs; buildPythonApplication rec {
+let
+  python3' = python3.override {
+    packageOverrides = self: super: {
+      sqlalchemy = super.sqlalchemy.overridePythonAttrs (oldAttrs: rec {
+        version = "1.3.24";
+        src = oldAttrs.src.override {
+          inherit version;
+          sha256 = "ebbb777cbf9312359b897bf81ba00dae0f5cb69fba2a18265dcc18a6f5ef7519";
+        };
+      });
+      sqlalchemy-utils = super.sqlalchemy-utils.overridePythonAttrs (oldAttrs: rec {
+        version = "0.36.6";
+        src = oldAttrs.src.override {
+          inherit version;
+          sha256 = "0srs5w486wp5zydjs70igi5ypgxhm6h73grb85jz03fqpqaanzvs";
+        };
+      });
+    };
+  };
+in
+with python3'.pkgs; buildPythonApplication rec {
   version = "4.5";
   pname = "buku";