This adds:
- Add ASYNC/AWAIT tokens back to Grammar and regenerate
- Recognize async/await keywords conditionally if feature_version < 7
- Reject await expressions if feature_version < 5
- Docs for ASYNC/AWAIT tokens and for ast.parse(..., feature_version=N)

Methods are always bound, and `__self__` can no longer be `NULL`
(`method_new()` and `PyMethod_New()` both explicitly check for this).

Moreover, once a bound method is bound, it *stays* bound and won't be re-bound
to something else, so the section in the datamodel that talks about accessing
an methods in a different descriptor-binding context doesn't apply any more in
Python 3.

For command line option, `option` is better than `parameter`.

Use UTF-8 as the system encoding on VxWorks.

The main reason are:

1. The locale is frequently misconfigured.
2. Missing some functions to deal with locale in VxWorks C library.

* bpo-20906: Corrections & revisions to Unicode HOWTO

* bpo-34484: don't describe range as a Private Use Area

The 3.8 docs claim that `os.posix_spawn` was introduced in 3.7, but it wasn't; it will be introduced in 3.8.

It is changed from 16KiB to 64KiB.  The previous default value
is used since 1990.

coreutils chose 128 KiB as minimum buffer size for block device I/O.

But shutil.copyfileobj() can be used for non block devices.
So I choose more conservative value.

As my quick benchmark, performance difference between 64KiB and
128 KiB is up to ~5%.  On the other hand, performance difference
between 32 KiB and 64 KiB can be more than 10% when file is fully
buffered.

This is why 64 KiB is rational value.

https://bugs.python.org/issue36043