passlib: Annotate various handler methods and fields (#7521)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

stubs/passlib/@tests/stubtest_allowlist.txt

@@ -29,3 +29,8 @@ passlib.utils.compat._ordered_dict
 
 # initialized to None, but set by concrete sub-classes
 passlib.handlers.pbkdf2.Pbkdf2DigestHandler.default_rounds
+passlib.utils.handlers.GenericHandler.setting_kwds
+
+# set to None on class level, but initialized in __init__
+passlib.utils.handlers.HasManyIdents.ident
+passlib.utils.handlers.HasRounds.rounds