MRG: 4.9.1 release branch, round 2 (#3666)

Whups. Fix #3665 by
updating nix version. Also, like, follow the release docs properly.

Author: ctb

flake.nix

@@ -63,7 +63,7 @@
 
           sourmash = python.buildPythonPackage ( commonArgs // rec {
             pname = "sourmash";
-            version = "4.9.0";
+            version = "4.9.1";
             format = "pyproject";
 
             cargoDeps = rustPlatform.importCargoLock {