This will make it easier to clean up channels (e.g. when used in tests).

Patch by Thomas Jones.

`obj.async` is now a syntax error, so the warning/shim is
quasi-unnecessary.

The editline emulation needs to be initialized *after* the name is
defined. This fixes the long open issue.

Only gcmodule.c uses _PyGC_REFS* macros now.
This makes easy to read GC code.

Add more tests for subinterpreters. This patch also fixes a few small defects in the channel implementation.

* test_ssl: Remove skip_if_broken_ubuntu_ssl

We no longer support OpenSSL 0.9.8.15.15.

* bpo-28167: Remove platform.linux_distribution

* bpo-24318: Rewrite the README PGO section.

Merged from a phone on an airplane. :)

This condition as already tested before this code is called.

To be more consistent with other code (and so people stop hassling me!), reflow the dataclasses comments to not use a single space indentation when continuing a paragraph of text.

This is an initial version that likely requires much polishing. I'm adding it lay out the structure and so we have something to start working from.