[3.13] gh-58956: Fix a frame refleak in bdb (GH-128190)

[miss-islington]

(cherry picked from commit 767c89b)

Co-authored-by: Tian Gao [email protected]

• Issue: Pdb does not stop at a breakpoint #58956

[gaogaotiantian]

Hi @iritkatriel , sorry this was supposed to be a backport but the test actually found another potential leak - we never clear curframe_locals after leaving pdb, which could have a reference to a frame and keep all the frames alive. I added one line to clear it in forget(), it is simple and it actually does similar things as other changes so I did mot make a separate PR. Could you take a quick look?

[iritkatriel]

Testing is useful!