gitea: Inherit from Gogs lister

This removes code and adds support for incremental pagination.

While both are essentially the same lister now, it still makes sense to
keep the Gitea lister separate, in order to:

1. display them in different categories on https://archive.softwareheritage.org/
2. support possible divergence of APIs in the future

swh/lister/gogs/lister.py

@@ -96,7 +96,8 @@ class GogsLister(Lister[GogsListerState, GogsListerPage]):
                     "Using authentication credentials from user %s", username or "???"
                 )
             else:
-                raise ValueError("No credentials or API token provided")
+                # Raises an error on Gogs, or a warning on Gitea
+                self.on_anonymous_mode()
 
         self.max_page_limit = 2
 
@@ -105,10 +106,15 @@ class GogsLister(Lister[GogsListerState, GogsListerPage]):
             {
                 "Accept": "application/json",
                 "User-Agent": USER_AGENT,
-                "Authorization": f"token {self.api_token}",
             }
         )
 
+        if self.api_token:
+            self.session.headers["Authorization"] = f"token {self.api_token}"
+
+    def on_anonymous_mode(self):
+        raise ValueError("No credentials or API token provided")
+
     def state_from_dict(self, d: Dict[str, Any]) -> GogsListerState:
         return GogsListerState(**d)