Fix return annotations of several methods that return self at runtime (#7070)

stdlib/pdb.pyi

@@ -1,5 +1,6 @@
 import signal
 import sys
+from _typeshed import Self
 from bdb import Bdb
 from cmd import Cmd
 from inspect import _SourceObjectType
@@ -173,4 +174,4 @@ def getsourcelines(obj: _SourceObjectType) -> tuple[list[str], int]: ...
 def lasti2lineno(code: CodeType, lasti: int) -> int: ...
 
 class _rstr(str):
-    def __repr__(self) -> _rstr: ...
+    def __repr__(self: Self) -> Self: ...