obsutil: maintain a homogenous list when computing successors In practice, this wasn't a problem because of an internal check for an empty list, but it avoids the following pytype error: File "/mnt/c/Users/Matt/hg/mercurial/obsutil.py", line 823, in successorsandmarkers: No attribute 'markers' on List[nothing] [attribute-error] In Union[Any, List[nothing], _succs] Differential Revision: https://phab.mercurial-scm.org/D10213

--- a/mercurial/obsutil.py	Fri Mar 12 23:28:56 2021 -0500
+++ b/mercurial/obsutil.py	Fri Mar 12 21:19:17 2021 -0500
@@ -782,7 +782,7 @@
     # closestsuccessors returns an empty list for pruned revisions, remap it
     # into a list containing an empty list for future processing
     if ssets == []:
-        ssets = [[]]
+        ssets = [_succs()]
 
     # Try to recover pruned markers
     succsmap = repo.obsstore.successors