lister: Make sure lister that requires github tokens can use it

Deploying the nixguix lister, I realized that even though the credentials configuration
is properly set for all listers, the listers actually requiring github origin
canonicalization do not have access to the github credentials. It's lost during the
constructor to only focus on the lister's credentials. Which currently translates to
listers being rate-limited.

This commit fixes it by pushing the self.github_session instantiation in the constructor
when the lister explicitely requires the github session. Hence lifting the rate limit
for maven, packagist, nixguix, and github listers.

Related to infra/sysadm-environment#4655

swh/lister/github/lister.py

@@ -11,7 +11,7 @@ from urllib.parse import parse_qs, urlparse
 
 import iso8601
 
-from swh.core.github.utils import GitHubSession, MissingRateLimitReset
+from swh.core.github.utils import MissingRateLimitReset
 from swh.scheduler.interface import SchedulerInterface
 from swh.scheduler.model import ListedOrigin
 
@@ -78,6 +78,7 @@ class GitHubLister(Lister[GitHubListerState, List[Dict[str, Any]]]):
             credentials=credentials,
             url=self.API_URL,
             instance="github",
+            with_github_session=True,
         )
 
         self.first_id = first_id
@@ -85,11 +86,6 @@ class GitHubLister(Lister[GitHubListerState, List[Dict[str, Any]]]):
 
         self.relisting = self.first_id is not None or self.last_id is not None
 
-        self.github_session = GitHubSession(
-            credentials=self.credentials,
-            user_agent=str(self.session.headers["User-Agent"]),
-        )
-
     def state_from_dict(self, d: Dict[str, Any]) -> GitHubListerState:
         return GitHubListerState(**d)
 
@@ -109,6 +105,7 @@ class GitHubLister(Lister[GitHubListerState, List[Dict[str, Any]]]):
             logger.debug("Getting page %s", current_url)
 
             try:
+                assert self.github_session is not None
                 response = self.github_session.request(current_url)
             except MissingRateLimitReset:
                 # Give up

swh/lister/github/tests/test_lister.py

@@ -141,7 +141,7 @@ def test_anonymous_ratelimit(swh_scheduler, caplog, requests_ratelimited) -> Non
     caplog.set_level(logging.DEBUG, "swh.core.github.utils")
 
     lister = GitHubLister(scheduler=swh_scheduler)
-    assert lister.github_session.anonymous
+    assert lister.github_session is not None and lister.github_session.anonymous
     assert "using anonymous mode" in caplog.records[-1].message
     caplog.clear()