Use a uniform User-Agent on all listers

This also adds tests to make sure that we properly send our version number to
upstreams.

swh/lister/npm/lister.py

@@ -69,8 +69,9 @@ class NpmListerBase(IndexingHttpLister):
         registry.
 
         """
-        return {'User-Agent': 'Software Heritage npm lister',
-                'Accept': 'application/json'}
+        headers = super().request_headers()
+        headers['Accept'] = 'application/json'
+        return headers
 
     def _compute_urls(self, repo_name):
         """Return a tuple (package_url, package_metadata_url)

swh/lister/npm/tests/test_tasks.py

@@ -40,7 +40,6 @@ def test_incremental(lister, seq, save, swh_app, celery_session_worker):
     # setup the mocked NpmLister
     lister.return_value = lister
     lister.run.return_value = None
-    lister.request_headers.return_value = []
     seq.return_value = 42
     save.side_effect = mock_save