I said "Bug" in the title because, while a blocked DNS is not a bug per se, what platformio does try to do here is definitely a bug:
platformio does NOT look for another mirror like it says in the output, instead it floods the DNS with hundreds of requests per second (while loop as fast as the CPU can).
it continiously tries sin1.contabostorage.com repeatedly.
my primary issue filing here is the above mentioned "DNS flood"-type bug.
on a secondary note: you probably want to fix your DNS (you are definitely on the blocklist of QUAD 9).