In https://docs.python.org/3/library/__future__.html:
annotations | 3.7.0b1 | *4.0* | PEP 563: Postponed evaluation of annotations

In PEP 563:
Starting with Python 3.7, a __future__ import is required to use the described functionality. No warnings are raised.

In Python *3.10* this will become the default behavior. Use of annotations incompatible with this PEP is no longer supported.

Python 4.0 or 3.10? Not clear.

The PEP was accepted apparently by Guido sometime after the second posting 21-Nov-2017.  Guido added the annotations entry to __future__ on 1/26/2018 with the 'mandatory' version changed to 4.0, which means 'some indefinite undecided future version'.  I consider '4.0' to the intentional and definitive until Guido or Lucasz say otherwise.

When PEP implementations differ from the PEP, we do not usually go back and revise the PEP.  The implementation rules unless clearly buggy.

The PEP now says 3.10, so the docs for __future__ should be fixed.

Ping?

Waiting for someone to submit a PR.

New changeset 0028c14073109595e7532ec00bb1e8bf39ecfb4d by YoSTEALTH in branch 'master':
bpo-41314: fixed annotations __future__ version (GH-21616)
https://github.com/python/cpython/commit/0028c14073109595e7532ec00bb1e8bf39ecfb4d

@ned This will need your assistance to merge the 3.7 backport IIUC.

Never mind, Miss Islington failed to backport to 3.7, so let's forget about that.

@gvanrossum I found couple of odd places where 4.0 is mentioned like https://docs.python.org/3/library/array.html for example, should a new issue be created? I am not sure how many more there are.

New changeset b99f770230e0db390aa80c5135b1053f3be48e19 by Miss Islington (bot) in branch '3.8':
bpo-41314: fixed annotations __future__ version (GH-21616)
https://github.com/python/cpython/commit/b99f770230e0db390aa80c5135b1053f3be48e19

New changeset fdc91c2e6096f60ddb8b7f72fe7926ccbbf42800 by Miss Islington (bot) in branch '3.9':
bpo-41314: fixed annotations __future__ version (GH-21616)
https://github.com/python/cpython/commit/fdc91c2e6096f60ddb8b7f72fe7926ccbbf42800

In general, "4.0" means "never", and for some deprecations that seems totally appropriate. I wouldn't worry about it. I'm closing this issue now that all three PRs have been merged. Thanks for your contribution!