This is not ideal as the releases are hardcoded, but I think is better
than nothing until we got a solution to automate this

cascache: use errno module

Closes #577

See merge request BuildStream/buildstream!674

os.errno does no longer work with Python 3.7

Closes #577.

Use http instead of https for the gnu ftp mirror

See merge request BuildStream/buildstream!676

https seems broken on the Debian image:

START   autotools/hello/42930621-fetch.499.log
START   Fetching https://ftpmirror.gnu.org/gnu/automake/automake-1.16.tar.gz
FAILURE Fetching https://ftpmirror.gnu.org/gnu/automake/automake-1.16.tar.gz
FAILURE tar source at hello.bst [line 16 column 2]: Error mirroring https://ftpmirror.gnu.org/gnu/automake/automake-1.16.tar.gz: <urlopen error [SSL: SSLV3_ALERT_HANDSHAKE_FAILURE] sslv3 alert handshake failure (_ssl.c:720)>

Trying to mitigate a file system issue

See merge request BuildStream/buildstream!595

This patch mitigates the granularity of the mtimes used on the default
gitlab runners, Allowing the test suite to pass on these runners.

Fix ostree repository mirroring

Closes #538

See merge request BuildStream/buildstream!658

Ostree mirrors were not sharing the same local repository, so it was
impossible the request refs from the right local repository when data
was fetched from a mirror rather than upstream.

Instead of having several repository with one remote each, we now
have one repository with several remotes.

This fixes #538.

Allow source plugins to access previous sources

Closes #381

See merge request BuildStream/buildstream!568