Bump Python to 3.8 on Windows

[christophebedard]

3.8 is the currently-targeted Python version for Windows on all currently-supported distro according to REP 2000 for:

• Humble: https://www.ros.org/reps/rep-2000.html#humble-hawksbill-may-2022-may-2027
• Iron: https://www.ros.org/reps/rep-2000.html#iron-irwini-may-2023-november-2024
• Jazzy: https://www.ros.org/reps/rep-2000.html#jazzy-jalisco-may-2024-may-2029