Update Python coding guidelines in copilot instructions

Added guidelines for maintaining source code readability and using idiomatic Python.

Author: bact

.github/copilot-instructions.md

@@ -193,6 +193,8 @@
 
 ## Python
 
+- [ ] Maintain source code readability.
+- [ ] Use Idiomatic Python.
 - [ ] Defensive coding: always check for None/empty and handle exceptions
       when dealing with external inputs, like function arguments,
       file I/O, network I/O, etc.