Bump python to 3.9

flake.nix

@@ -21,7 +21,7 @@
           devShells.default = pkgs.mkShell {
             buildInputs = [
               (mach-nix.mkPython {
-                python = "python37";
+                python = "python39";
                 requirements = lib.concatStringsSep "\n" [
                   (builtins.replaceStrings [ "mysqlclient==" ]
                     [ "mysqlclient>=" ] (builtins.readFile ./requirements.txt))