Add a pyproject.toml file

[SamStephens]

I've just noticed that my installs of ratelimit show the message:

DEPRECATION: ratelimit is being installed using the legacy 'setup.py install' method, because it does not have a 'pyproject.toml' and the 'wheel' package is not installed. pip 23.1 will enforce this behaviour change. A possible replacement is to enable the '--use-pep517' option. Discussion can be found at pypa/pip#8559

It seems that ratelimit should probably add a pyproject.toml file to meet modern best practices.