python3: switch to python38 for 20.09

[FRidh]

Describe the bug
Use python38 as python3 for 20.03.

1. Let Hydra build python38Packages
2. If sufficient packages work for 20.03 we can switch, otherwise we remain at 3.7.

[worldofpeace]

Also, a call to python package maintainers, we need to make sure your packages work with python38 for 20.03.

If you have PRs fixing this please reference this issue, or if you open an issue to reference it also.
That way we can track the status of this by viewing one long-running thread, or you can be notified here.

[jonringer]

For cross reference: #71222

[FRidh]

2dc4ab3

[jonringer]

@FRidh There's still plenty of the python ecosystem which doesn't support 3.8. Personally, I think this should have waited another month.

[FRidh]

Looking at python37Packages and python38Packages there wasn't much of a difference. Given the upcoming 20.03 release I do not want to wait another month with making such change or it would get too close to branch-off. Now we can see how many applications are compatible, and if its too few, we can always revert before branch-off.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/nixos-20-03-feature-freeze/5655/1

[FRidh]

This was reverted.

If we manage to have another staging cycle before branch-off, then we could give it another try, otherwise, we should postpone this to 20.09.

[jonringer]

There was quite a bit of deprecation that took place in python3.8. Personally I'm fine with floating python3 as python3.7 until 20.09

[stale]

Hello, I'm a bot and I thank you in the name of the community for opening this issue.

To help our human contributors focus on the most-relevant reports, I check up on old issues to see if they're still relevant. This issue has had no activity for 180 days, and so I marked it as stale, but you can rest assured it will never be closed by a non-human.

The community would appreciate your effort in checking if the issue is still valid. If it isn't, please close it.

If the issue persists, and you'd like to remove the stale label, you simply need to leave a comment. Your comment can be as simple as "still important to me". If you'd like it to get more attention, you can ask for help by searching for maintainers and people that previously touched related code and @ mention them in a comment. You can use Git blame or GitHub's web interface on the relevant files to find them.

Lastly, you can always ask for help at our Discourse Forum or at #nixos' IRC channel.

[FRidh]

This has been done. Need to check the documentation and changelog.

[schmittlauch]

@FRidh From the 20.03 changelog:

Python 3 now defaults to Python 3.8 instead of 3.7.

So I guess this can be closed.