Add tool.setuptools:py-modules to pyproject.toml file

[PinkShnack]

As it came up in another PR (#146), I couldn't install shapeout2 with pip install -e .[all] because of pyproject searching for project names, and taking e.g. "venv311" as a possible package name alongside "shapeout2". See here: #146 (comment)

This fixes the issue for me:

[tool.setuptools]
py-modules = ['shapeout2']

but as mentioned in the above link, apparently naming your environment ".env" or having the env outside the repo will also fix the issue. I don't think that's very realistic. I have multiple environments, and naturally keep track of them by naming them "venv3XY" e.g. "venv39", "venv312".

Also, PyCharm's default is "venv".