Buildstream tar source fails when receiving HTTP code 103

Summary

One server we're dealing with returns HTTP code 103 "Early Hints", and bst is treating that as an error.

Steps to reproduce

try tracking core-deps/ppp.bst or core-deps/samba.bst on gnome-build-meta commit 3e945aab3d01fe07dbe7da8093cab82de71deb63

What is the current bug behavior?

I get an error https://gitlab.gnome.org/GNOME/gnome-build-meta/-/jobs/421663

What is the expected correct behavior?

It should continue normally, HTTP codes in the 100 range are informational

Possible fixes

https://gitlab.com/BuildStream/buildstream/blob/master/src/buildstream/plugins/sources/_downloadablefilesource.py#

It seems urllib is treating HTTP code 103 as an error.

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.4

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