apbs: fix py311 build

https://github.com/Electrostatics/apbs/blob/c2e5af22c9044b2bb8f9c1114710add58ec5b007/tools/manip/inputgen.py#L314

pkgs/applications/science/chemistry/apbs/default.nix

···
     substituteInPlace CMakeLists.txt \
       --replace "include(ImportFETK)" "" \
       --replace 'import_fetk(''${FETK_VERSION})' ""
+
+    # U was removed in python 3.11 because it had no effect
+    substituteInPlace tools/manip/inputgen.py \
+      --replace '"rU"' '"r"'
   '';
 
   nativeBuildInputs = [