[Feature] Migrate to pyproject.toml

[mikealfare]

resolves #
docs dbt-labs/docs.getdbt.com/#

Problem

dbt-snowflake currently uses setup.py and a collection of independent configs to build and test. This has been replaced by pyproject.toml as the go-forward config file for python projects. Additionally, there are configs that live in multiple files and differ from each other, leading to inconsistent usage of dev and test tools.

Solution

Config will be migrated as much as possible into pyproject.toml. There are still some projects that haven't migrated to PEP-518, namely pre-commit, flake8, and tox. These will need to live in legacy formats for now. tox and flake8 can only read ini, so they will live in setup.cfg. pre-commit can only read yaml, so it will remain in place, in .pre-commit-config.yaml.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the dbt-snowflake contributing guide.

[mikealfare]

I got too far ahead on this PR. This will be broken down into smaller PRs, such as #835.