Increase flake8

[cthoyt]

Summary

This PR adds more flake8 checks and better standardizes applying black with a line length of 120.

• [x] Code passes all tests
• [x] Unit tests provided for these changes
• [x] Documentation and docstrings added for these changes

Changes

• Add black configuration in pyproject.toml
• Enable all flake8 tests by removing the --select BLK120,I001 flag
• Ensure tox's run of flake8 has the right line length with the --max-line-length 120 flag
• Apply black (not sure why this wasn't being picked up before :/)
• Code cleanup (remove unused imports, fix literals, add noqa for some imports [for now])