Add pyproject.toml

[benjaminrwilson]

This PR adds a pyproject.toml file to consolidate styling tools such as isort and black.

• pyproject.toml was introduced in PEP 518 for build system requirements.
• Allows for a unified configuration file for a host of tools, e.g. isort, black, etc.

Each "block" within this file specifies a different type of options. For example, isort is written as:

[tool.isort]

known_first_party = "argoverse"
line_length = 120
multi_line_output = 3
profile = "black"

The options above should have a 1-to-1 correspondence with isort.cfg.

[johnwlambert]

Ben, what are the advantages of pyproject.toml over our current repo setup?