fix treating LsRemoteResult as a dictionary is deprecated (#449)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: shcheklein

src/scmrepo/git/backend/dulwich/__init__.py

@@ -601,14 +601,13 @@ def iter_remote_refs(self, url: str, base: Optional[str] = None, **kwargs):
             raise InvalidRemote(url) from exc
 
         try:
+            refs = client.get_refs(path.encode()).refs
+
             if base:
-                yield from (
-                    os.fsdecode(ref)
-                    for ref in client.get_refs(path.encode())
-                    if ref.startswith(os.fsencode(base))
-                )
+                base_b = os.fsencode(base)
+                yield from (os.fsdecode(ref) for ref in refs if ref.startswith(base_b))
             else:
-                yield from (os.fsdecode(ref) for ref in client.get_refs(path.encode()))
+                yield from (os.fsdecode(ref) for ref in refs)
         except NotGitRepository as exc:
             raise InvalidRemote(url) from exc
         except HTTPUnauthorized as exc: