Deprecate WS_1004_NO_STATUS_RCVD and WS_1005_ABNORMAL_CLOSURE

[Kludex]

Closes #1578

This PR has the same idea as django/asgiref#272.

The idea here is to deprecate two constants using the PEP 562. Unfortunately I need to use a backport for Python 3.6 - but I guess we'd want to release this for Python 3.6 before removing the Python 3.6 support.

NOTE: After this PR gets in, a patch release is needed.

[aminalaee]

@Kludex Is this worth the effort as Python 3.6 is already EOL and there's plan to remove support in Starlette too?
I mean as you said we can hold it after Python 3.6 is removed.

[Kludex]

I mean as you said we can hold it after Python 3.6 is removed.

I didn't say that, at least I didn't mean it. I mean that we should fix it before stop supporting Python 3.6, so at least the last version has it right.

[euri10]

looks good to me, thanks @Kludex

[adriangb]

So the idea here is to cut 1 release with those code then immediately remove it right?

So:

1. Merge this PR. Cut a patch release w/ Python 3.6 support.
2. New PR that deletes the back port code.
3. Cut a minor release dropping Python 3.6 support.

Am I getting this right?

[Kludex]

Yes.

[Kludex]

The backport removal will be on the PR that drops Python 3.6.