support/scripts/pkg-stats: use aiohttp for latest version retrieval

support/scripts/pkg-stats: use aiohttp for latest version retrieval

This commit reworks the code that retrieves the latest upstream
version of each package from release-monitoring.org using the aiohttp
module. This makes the implementation much more elegant, and avoids
the problematic multiprocessing Pool which is causing issues in some
situations.

Since we're now using some async functionality, the script is Python
3.x only, so the shebang is changed to make this clear.

Suggested-by: Titouan Christophe <titouan.christophe@railnova.eu>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>