Move pyproject.toml to root directory

[mattt]

This PR moves pyproject.toml from the /python subdirectory to the repository root. Python tools and editors expect pyproject.toml to be located at the root directory, even if the package source is located in a subdirectory.

This change has several benefits:

• It simplifies how Python tools for testing and linting are installed and run
• It lets us consolidate mypy configuration into pyproject.toml
• It allows us to remove symlinks to README and LICENSE (which are distributed with the Python package; now they're included directly)