Configure formatting in pyproject.toml; set required python to >=3.8

[RalfG]

Added

• isort and black configurations in pyproject.toml (with max line length of 120)
• Black check in GitHub Actions
• New CI workflow with Flake8 linting and Black checks for code in ./streamlit

Changed

• Removed support for Python 3.7 (end of life), added 3.11 to CI checks

Removed

• Removed some unused imports

Note: Two new CI checks are failing because the code base has not been formatted with Black yet.

[enryH]

Let's update the formatting continuously on the other branches and gradually move to black. I think this should not fail by tomorrow.

[enryH]

Suggested change

	target-version = ['py38']
	target-version = ["py38", "py39", "py310", "py311"]