style: Replace single quotes with double quotes in main.py

Author: paul-gauthier

aider/main.py

@@ -1110,7 +1110,7 @@ def get_io(pretty):
             coder.ok_to_warm_cache = False
 
             # Set the placeholder if provided
-            if hasattr(switch, 'placeholder') and switch.placeholder is not None:
+            if hasattr(switch, "placeholder") and switch.placeholder is not None:
                 io.placeholder = switch.placeholder
 
             kwargs = dict(io=io, from_coder=coder)