[SingletonQuery] correct superclass typing

I had misinterpreted the pattern type for this query to be 'NoReturn',
thinking that it accepted no pattern at all (even so, I should have
typed it as 'None').  However, it does accept a pattern, and uses it to
decide whether to select only singletons or none at all.

See <beetbox#5282 (comment)>.

beets/library.py

@@ -23,7 +23,6 @@
 import time
 import unicodedata
 from functools import cached_property
-from typing import NoReturn
 
 from mediafile import MediaFile, UnreadableFileError
 
@@ -50,7 +49,7 @@
 # Library-specific query types.
 
 
-class SingletonQuery(dbcore.FieldQuery[NoReturn]):
+class SingletonQuery(dbcore.FieldQuery[str]):
     """This query is responsible for the 'singleton' lookup.
 
     It is based on the FieldQuery and constructs a SQL clause
@@ -61,7 +60,7 @@ class SingletonQuery(dbcore.FieldQuery[NoReturn]):
     and singleton:false, singleton:0 are handled consistently.
     """
 
-    def __new__(cls, field, value, *args, **kwargs):
+    def __new__(cls, field: str, value: str, *args, **kwargs):
         query = dbcore.query.NoneQuery("album_id")
         if util.str2bool(value):
             return query