gh-132869: Fix crash in `_PyObject_TryGetInstanceAttribute` (#133700)

This fixes a crash in `_PyObject_TryGetInstanceAttribute` due to the use
of `_PyDictKeys_StringLookup` on an unlocked dictionary that may be
concurrently modified.

The underlying bug was already fixed in 3.14 and the main branch.

(partially cherry picked from commit 1b15c89a17ca3de6b05de5379b8717e9738c51ef)