BLD: Add pyproject.toml
* Adds pyproject.toml to support this.

* Bumps minimum Cython version to the version supporting Python 3.8.

Closes https://github.com/pandas-dev/pandas/issues/20775
xref https://github.com/pandas-dev/pandas/issues/27435 (check if closes)
xref https://github.com/pandas-dev/pandas/pull/25227, https://github.com/pandas-dev/pandas/pull/20718, https://github.com/pandas-dev/pandas/pull/16745