Modernize setuptools usage in pyproject.toml 1. Remove the redundant `wheel` dependency. The setuptools build backend has been adding it automatically since day one, and it was explicitly specified in the docs as a mistake. See: https://github.com/pypa/setuptools/commit/f7d30a9529378cf69054b5176249e5457aaf640a 2. Replace the legacy backend with the regular backend. The legacy backend was only intended to be used implicitly when `pyproject.toml` does not specify only, and was not supposed to be specified explicitly there. See: https://github.com/pypa/setuptools/issues/1689 3. Prepend the current directory to `sys.path` as required for `setup.py` to reliably import `setupbase`. The non-legacy backend no longer does this for us.