Added possibility to use specific python in writePythonApplication.

This feature can be useful to add script dependant dependencies
using pkgs.python3.withPackages.

flake.nix

@@ -12,18 +12,20 @@
         pkgs = import nixpkgs { inherit system;};
       in
     {
-      writePythonApplication = {name, contentFile, extraRuntimeInputs ? []}: let
+      writePythonApplication = {name, contentFile, extraRuntimeInputs ? [], python ? null}: 
+          let
+            python-to-use = python == null then pkgs.python311 else python;
             wrappedScript = pkgs.writeText "wrappedScript" (builtins.readFile contentFile);
-            in
+          in
             pkgs.writeShellApplication {
-            name = name;
-            runtimeInputs = [
-                wrappedScript
-                pkgs.python311] ++ extraRuntimeInputs;
-            text = ''
-                exec ${pkgs.python38}/bin/python ${wrappedScript} "$@"
-            '';
-            };
+              name = name;
+              runtimeInputs = [
+                  wrappedScript
+                  if python-to-use] ++ extraRuntimeInputs;
+              text = ''
+                  exec ${python-to-use}/bin/python ${wrappedScript} "$@"
+              '';
+          };
 
         writeBabashkaApplication = {name, contentFile, extraRuntimeInputs ? []}: let
             wrappedScript = pkgs.writeText "wrappedScript" (builtins.readFile contentFile);