"requests" library supports HTTP2, HTTPS, coockies by default and
connection:keep-alive header, which increase the number of positive responses
from the probed servers.
Nevertheless, due to unreliable nature of the Internet and flaky performance of
web servers some links occasionaly fail, and with this many links to
probe (1.3k+) it becomes very likely that at least one of them fails.
To mitigate these failures, the script retries the failed links once more.
