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