I can confirm that this also happens in python-pip and python3-pip. I found one work around that seems to work and that is to add a timeout, e.g.
pip3 --default-timeout=1000 install requests I also found reports that this is ipv6 related. Better to just upgrade to v18+ as is recommended in https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=901393
