python3Packages.cynthion: relax pygreat dependency

Upstream pins pygreat~=2024.0.5, but cynthion 0.2.4 works fine against
pygreat 2026.0.0; drop the constraint so the package builds against
the current pygreat.

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

@@ -51,6 +51,8 @@ buildPythonPackage rec {
     setuptools
   ];
 
+  pythonRelaxDeps = [ "pygreat" ];
+
   pythonRemoveDeps = [ "future" ];
 
   dependencies = [