Make the dependency of 'pytest_plugin' on 'requests' optional.
Else, running 'pytest' always crashes if 'swh.core' is installed but 'requests' isn't.
Migrated from D4756 (view on Phabricator)
Staging instance, all changes can be removed at any time
Else, running 'pytest' always crashes if 'swh.core' is installed but 'requests' isn't.
Migrated from D4756 (view on Phabricator)