swh-core: missing test dependency on requests

In a brand new python module that depends only on swh-core, pytest fails as:

  File "/home/zack/dati/projects/sw-heritage/git/swh-environment/swh-fuse/.tox/py3/lib/python3.8/site-packages/swh/core/pytest_plugin.py", line 9, in <module>
    import requests
ModuleNotFoundError: No module named 'requests'

and indeed that plugin depends on requests.

I think this means that an explicit test-time dependency on requests is missing from swh-core.

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Migrated from T2634 (view on Phabricator)