Update the install section with 3.9 after October 1st

[pushfoo]

TL;DR: The "Installing Python" section uses 3.8, which'll EOL when Python 3.13 releases in October 2024

Where?

On the site / Steps To Replicate

1. Go to https://learn.arcade.academy/en/latest/chapters/02_setup/setup.html

2. Scroll "Installing Python" (or click here)

3. Observe the page shows 3.8.X in two screnshots:

   1. In the image showing how to add Python to PATH
   2. In the subsequent image:
      

In the code

The lines which include these images are here:

arcade_book/source/chapters/02_setup/setup.rst

Lines 100 to 107 in b82e980

	.. image:: setup_windows_1.png
	:width: 450px
	After that, you can just close the dialog. There's no need to increase the path length,
	although it doesn't hurt anything if you do.
	.. image:: setup_windows_5.png
	:width: 450px