intermittent jenkins failures for tor-ci

There seem to be intermittent Jenkins failures for some tor-ci-linux-master builds, related to timeouts retrieving packages. Some examples are below. network-team: please note additional similar failures on this ticket when they appear.

https://jenkins.torproject.org/view/Failed+Unstable/job/tor-ci-linux-master/ARCHITECTURE=i386,SUITE=stretch/4022/console

16:24:13 E: Failed to fetch https://mirrors.wikimedia.org/debian/pool/main/p/python2.7/libpython2.7-stdlib_2.7.13-2+deb9u3_i386.deb  Operation too slow. Less than 10 bytes/sec transferred the last 120 seconds
16:24:13 E: Some files failed to download
16:24:13 run-parts: /home/jenkins/jenkins-tools/slaves/linux/tor-ci-linux-master/setup/15-install-build-depends exited with return code 1

https://jenkins.torproject.org/view/Failed+Unstable/job/tor-ci-linux-master-expensive-hardening/ARCHITECTURE=amd64,SUITE=buster/2447/console

18:41:41 E: Failed to fetch https://mirrors.wikimedia.org/debian/pool/main/p/python2.7/libpython2.7-minimal_2.7.16-2_amd64.deb  Could not wait for server fd - select (11: Resource temporarily unavailable) [IP: 2620:0:861:1:208:80:154:15 443]
18:41:41 E: Some files failed to download
18:41:41 run-parts: /home/jenkins/jenkins-tools/slaves/linux/tor-ci-linux-master-expensive-hardening/setup/15-install-build-depends exited with return code 1

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