Move packaging metadata to pyproject

[ordabayevy]

Summary

• Move project metadata, dependencies, extras, package discovery, and pytest/isort config into pyproject.toml.
• Move flake8 config into .flake8 because flake8 does not read pyproject.toml natively.
• Remove legacy setup.py and setup.cfg.

Test plan

• python -m build
• python -m pip install -e .
• pytest -q test/test_import.py

Breaking down #615 into smaller PRs