Fix pyproject.toml for the final release of bdpy supporting Python 3.6–3.8

[HirokiYasuda03]

Problem

latest version of bdpy doesn't support all the python version written in pyproject.toml {3.6>=, <3.12}.

Solution

To drop EOL python versions and release new bdpy that supports only python 3.9+, I propose to release final bdpy supporting Python 3.6~3.8.
To do this, I will fix the pyproject.toml and some bugs that change might cause.