Fix the following bugs in the C implementation:

* get_future_loop() silenced all exceptions raised when look up the get_loop
  attribute, not just an AttributeError.
* enter_task() silenced all exceptions raised when look up the current task,
  not just a KeyError.
* repr() was called for a borrowed link in enter_task() and task_step_impl().
* str() was used instead of repr() in formatting one error message (in
  Python implementation too).
* There where few reference leaks in error cases.

The proactor event loop has a race condition when reading with
pausing/resuming. `resume_reading()` unconditionally schedules the read
function to read from the current future. If `resume_reading()` was
called before the previously scheduled done callback fires, this results
in two attempts to get the data from the most recent read and an
assertion failure. This commit tracks whether or not `resume_reading`
needs to reschedule the callback to restart the loop, preventing a
second attempt to read the data.

A DeprecationWarning was emitted in Python 3.6-3.7.

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.