fix packbits extension bug

This PR is meant to resolve the #5 , again. It appears to be related to a version conflict between `Cython` and `Python` versions. - **related issue** - [#setuptools #3891](https://github.com/pypa/setuptools/issues/3891) This error seems to be occurring in several OS. - **related issue** - [macOS](https://github.com/jhj0517/stable-diffusion-webui-Layer-Divider/issues/5#issuecomment-1505081569) - [Windows](https://github.com/jhj0517/stable-diffusion-webui-Layer-Divider/issues/5#issuecomment-1514388605) When running `pip install pytoshop` in Python 3.6, the packbits extension file (`.so` or `.pyd`) is generated However for Python versions 3.7 ~ 3.11, it is not generated The action of `pip` should not differ from that of `setup.py`. However, when using `setup.py`, the so and pyd files are generated normally. - **related issue** - [#pytoshop 9](https://github.com/mdboom/pytoshop/issues/9#issuecomment-425882128) So I assume this error is related to a version conflict between `Cython` and `Python` versions. In this PR, I've directly built the extension.