It's necessary for the core tests to pass.
It's an optional dependency installed on the jenkins and production nodes.
It must be installed on developer machine otherwise errors will happen.
context:
```
10:20 <leni> Hi I have problems with tests on swh-core with test_uncompress_tarpaths. Do any of you have a hint ?
10:26 <+olasd> leni: please paste the command that you've run and a full traceback to a pastebin
10:29 <leni> It's with tox
10:29 <leni> https://pastebin.com/MVq9jJnc
... (edit)
> raise ValueError(f'Problem during unpacking {tarpath}. Reason: {e}')
E ValueError: Problem during unpacking /home/leni/swh-environment/swh-core/.tox/py3-core/lib/python3.6/site-packages/swh/core/tests/data/archives/hello.tar.lz. Reason: Unable to uncompress /home/leni/swh-environment/swh-core/.tox/py3-core/lib/python3.6/site-packages/swh/core/tests/data/archives/hello.tar.lz to /tmp/pytest-of-leni/pytest-36/test_uncompress_tarpaths0. Reason: Command '['tar', 'xf', '/home/leni/swh-environment/swh-core/.tox/py3-core/lib/python3.6/site-packages/swh/core/tests/data/archives/hello.tar.lz', '-C', PosixPath('/tmp/pytest-of-leni/pytest-36/test_uncompress_tarpaths0')]' returned non-zero exit status 2.
... (end)
10:40 <+ardumont> leni: install the lzip package?
10:41 <+ardumont> (it's not a package that tox can install through pip)
10:41 <+ardumont> .oO(it's missing from the doc https://docs.softwareheritage.org/devel/developer-setup.html#install-required-dependencies)
10:42 <+ardumont> D2635 should fix that
10:42 -- Notice(swhbot): D2635 (author: ardumont, Needs Review) on swh-docs: docs/developer-setup: Add missing lzip dependency <https://forge.softwareheritage.org/D2635>
10:43 <leni> Ok thanks
10:44 <+ardumont> did it work?
10:44 <leni> Yes it worked !
```