Some maintenance items: python 3.6/7 support, python 3.12 tests, setup.py and pyproject.toml updates

[jmahlik]

Before you request a new feature, please confirm:
[x] I have read through README and understand how the plugin works currently.
[x] I understand that this project is not actively adding new features, and there are no promises if/when my request will be implemented.
[x] (Optional) I am interested in working to add this feature through a PR.

What would you like to see added to this plugin?
I noticed there isn't a wheel on pypi. Wheels are quicker to install since recent versions of pip will build a wheel from the sdist and install the wheel if there isn't one on pypi.

When I was building the project using python -m build I noticed the project wouldn't be installable since it imports itself in the setup.py before being installed. So updated the setup.py to get the version from the file and not import flake8_use_fstring (somewhat related to #30, very similar error).

https://packaging.python.org/en/latest/guides/single-sourcing-package-version/

Also added Python 3.12 tests and dropped the tests for 3.6 and 3.7 since they are end of life python versions.

What is your rationale/ use case for this feature?
General maintenance.

What alternative solutions have you considered? Why do you prefer the solution you proposed?
It would be great to have a wheel and ensure the project keeps being installable in the future since virtualenvs in python 3.12 no longer include setuptools by default.

Additional relevant information: