Issue #19744: improve ensurepip error when ssl is missing
Removed spaces before commas and periods.
Removed spaces before commas and periods.
Make summary entry format consistent (always end with pep or issue in parens).
Add link to selectors description from its summary line.
Add pep 435 summary entry in new required features, and ensurepip description.

Also added a note about platform packagers not being required to install
pip by default, if they provide an appropriate pip hook, to the PEP
description section.