Duplicated `isearch_add_character` in `_pyrepl/historical_reader.py`

[tanloong]

There are two isearch_add_character in the __post_init__ method:

https://github.com/python/cpython/blob/main/Lib/_pyrepl/historical_reader.py#L242-L244

Linked PRs

• gh-145394: Remove duplicated isearch_add_character in _pyrepl/historical_reader.py #145396