New port: devel/py-selectors2

Backported, durable, and portable selectors for Python designed to replace
the standard library selectors module.

This module was originally written for the urllib3 project and later
released as an independent project.

PR: 230873
Submitted by: John W. O'Brien <john@saltant.com>
Reviewed by: mat (mentor)
Approved by: mat (mentor)
Differential Revision: https://reviews.freebsd.org/D17182