README Update Suggestion

Hello, I've been working with BitstreamEvolution for a bit and wanted to note a possible inconsistency with the README, as it may help future people who use the repo.

Specifically, the README states that BitstreamEvolution requires Python version 3.7 or higher. Personally, I encountered an error when running evolve.py when using Python 3.8:

This was thrown for the Circuit.py file, line 33

in Circuit def get_bitstream(self) -> list[int]: TypeError: 'type' object is not subscriptable

After looking up the issue, it seemed like this is a type hinting feature only supported by Python version 3.9 and higher. This was confirmed when the issue went away for me after switching to Python 3.9. Would making this change to the README be helpful?

README の Python バージョン要件を確認し、evolve.py を Python 3.8 で実行したときに Circuit.py の 33 行目で報告される型アノテーションと比較します。README がサポート対象の Python の最小バージョンを正確に記載し、確認された要件を反映していれば完了です。

索引モデルが issue の本文から書いたものです。