Add pyproject.toml (#60408)
Summary:
This makes PyTorch conform to [PEP 517](https://www.python.org/dev/peps/pep-0517/) and [PEP 518](https://www.python.org/dev/peps/pep-0518/) by explicitly stating that we use [`setuptools`](https://setuptools.readthedocs.io/). It also follows up on https://github.com/pytorch/pytorch/pull/60119#pullrequestreview-685791812 by moving our [`isort`](https://pycqa.github.io/isort/) config into the new `pyproject.toml` file. I didn't move any of our other tool configs into `pyproject.toml` in this PR because:
- `.flake8` is assumed to exist in its current format for `tools/actions_local_runner.py` to work
- `mypy.ini` is not our only `mypy` config
- `pytest.ini` has detailed comments on `addopts` which [would have to be removed](https://github.com/toml-lang/toml/issues/340#issuecomment-122164501) in TOML because that setting is [a string, not an array](https://docs.pytest.org/en/6.2.x/customize.html#pyproject-toml)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/60408
Reviewed By: 1ntEgr8
Differential Revision: D29277327
Pulled By: samestep
fbshipit-source-id: 3f2e63f6cf9024f8c534cb13a0d854a75609c5ba
