Second pass for asyncio subprocess and sync docs.


https://bugs.python.org/issue33649

This test is doesn't work when the test process is privledged, which is hard to detect.





https://bugs.python.org/issue34668

This function was not in any .h file and was not used by Python, so removing it is safe.


https://bugs.python.org/issue34653

[bpo-34658](https://www.bugs.python.org/issue34658): Fix a rare interpreter unhandled exception state SystemError only
seen when using subprocess with a preexec_fn while an after_parent handler has
been registered with os.register_at_fork and the fork system call fails.



https://bugs.python.org/issue34658

This causes the tearDown code to only unimport the test modules specifically created as part of each test via the self.mkhier method rather than abusing test.support.modules_setup() and the scary test.support.modules_cleanup() code.


https://bugs.python.org/issue34200

A fix for 883702ebb8bbfa749ef0040d1b58d6222bf589ee.

closes bpo-34650: Check if sched_getscheduler returns ENOSYS before declaring it supported. (GH-9228)

musl doesn't support the scheduler API, but declares stubs that alway return ENOSYS.

Reported by Svace static analyzer.

Returning EINTR from pthread semaphore or lock acquisition is an optional POSIX
feature. musl does not provide this feature, so some threadsignal tests fail
when Python is built against it.

There's no good way to test for musl, so we skip if we're on Linux and not using
glibc pthreads.

Also, hedge in the threading documentation about when we can provide interrupts
from lock acquisition.

* raise exception if _missing_ returns None or invalid type

Store a weak reference to stream readerfor breaking strong references

It breaks the strong reference loop between reader and protocol and allows to detect and close the socket if the stream is deleted (garbage collected)

* Fix enum members getting shadowed by parent attributes
* Move Enum._convert to EnumMeta._convert_
* Deprecate _convert

Improve Enum docs.


https://bugs.python.org/issue33437

In distutils.command.install, replace "pliant children" (previously,
it was "pliant slaves") with "helpers".

<!-- issue-number: [bpo-34605](https://www.bugs.python.org/issue34605) -->
https://bugs.python.org/issue34605
<!-- /issue-number -->

It is unused.

<!--
Thanks for your contribution!
Please read this comment in its entirety. It's quite important.

# Pull Request title

It should be in the following format:

```
bpo-NNNN: Summary of the changes made
```

Where: bpo-NNNN refers to the issue number in the https://bugs.python.org.

Most PRs will require an issue number. Trivial changes, like fixing a typo, do not need an issue.

# Backport Pull Request title

If this is a backport PR (PR made against branches other than `master`),
please ensure that the PR title is in the following format:

```
[X.Y] <title from the original PR> (GH-NNNN)
```

Where: [X.Y] is the branch name, e.g. [3.6].

GH-NNNN refers to the PR number from `master`.

-->