Buildstream tar source fails when receiving HTTP code 103

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1124)
In GitLab by [[Gitlab user @abderrahimk]](https://gitlab.com/abderrahimk) on Sep 7, 2019, 07:14

## 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

[//]: # (Add the BuildStream version. Substitute below the "x" by "1", "2" or the right version. Add more than one version if necessary)

* BuildStream version affected: /milestone %BuildStream_v1.4

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Start in src/buildstream/plugins/sources/_downloadablefilesource.py and reproduce the failure by tracking core-deps/ppp.bst or core-deps/samba.bst at the cited gnome-build-meta commit. Inspect how urllib handles HTTP 103 responses; done means tracking continues normally when an informational 100-range response is received.

Written by the indexing model from the issue text.