Update value of User-Agent HTTP request header used by listers

That HTTP header value will now contain the lister name but also a link
to our contact form in order for sysadmins to easily reach us if needed.

The following template is used to generate it:

"Software Heritage <lister_name> lister v<swh-lister version>
 (+https://www.softwareheritage.org/contact)"

swh/lister/pubdev/lister.py

@@ -12,15 +12,8 @@ from requests.exceptions import HTTPError
 from swh.scheduler.interface import SchedulerInterface
 from swh.scheduler.model import ListedOrigin
 
-from .. import __version__
 from ..pattern import CredentialsType, StatelessLister
 
-# https://github.com/dart-lang/pub/blob/master/doc/repository-spec-v2.md#metadata-headers
-USER_AGENT = (
-    f"Software Heritage PubDev Lister v{__version__} "
-    "(+https://www.softwareheritage.org/contact)"
-)
-
 logger = logging.getLogger(__name__)
 
 # Aliasing the page results returned by `get_pages` method from the lister.
@@ -51,12 +44,7 @@ class PubDevLister(StatelessLister[PubDevListerPage]):
             url=self.BASE_URL,
         )
 
-        self.session.headers.update(
-            {
-                "Accept": "application/json",
-                "User-Agent": USER_AGENT,
-            }
-        )
+        self.session.headers.update({"Accept": "application/json"})
 
     def get_pages(self) -> Iterator[PubDevListerPage]:
         """Yield an iterator which returns 'page'

swh/lister/pubdev/tests/test_lister.py

@@ -3,7 +3,8 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
-from swh.lister.pubdev.lister import USER_AGENT, PubDevLister
+from swh.lister import USER_AGENT_TEMPLATE
+from swh.lister.pubdev.lister import PubDevLister
 
 expected_origins = {
     "https://pub.dev/packages/Autolinker",
@@ -29,7 +30,10 @@ def test_pubdev_lister(datadir, requests_mock_datadir, swh_scheduler):
 
 
 def _match_request(request):
-    return request.headers.get("User-Agent") == USER_AGENT
+    return (
+        request.headers.get("User-Agent")
+        == USER_AGENT_TEMPLATE % PubDevLister.LISTER_NAME
+    )
 
 
 def test_pubdev_lister_skip_package(