BLD: update build requirement for py39 #37135
- [x] closes #37135 


> Many thanks for considering my request. But I'm really new with programming and cmd. How can I apply this code when I pip install in cmd?
> 
> ```
> "wheel",
> "Cython>=0.29.21,<3",  # Note: sync with setup.py
> "numpy==1.16.5; python_version=='3.7' and platform_system!='AIX'",
> "numpy==1.17.3; python_version>='3.8' and platform_system!='AIX'",
> "numpy==1.17.3; python_version=='3.8' and platform_system!='AIX'",
> "numpy==1.16.5; python_version=='3.7' and platform_system=='AIX'",
> "numpy==1.17.3; python_version>='3.8' and platform_system=='AIX'",
> "numpy==1.17.3; python_version=='3.8' and platform_system=='AIX'",
> "numpy; python_version>='3.9'",
> ```
> 
> ]
> 
> [tool.black]
> target-version = ['py37', 'py38']
> target-version = ['py37', 'py38', 'py39']
> exclude = '''
> (

I'll reply in the issue. thanks @fangchenli  Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

```
$ git checkout 1.1.x
$ git pull
```

2. Cherry pick the first parent branch of the this PR on top of the older branch:
```
$ git cherry-pick -m1 58dcafadfa50bb160afb2657a4fbda326d4802a6
```

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

```
$ git commit -am 'Backport PR #37144: BLD: update build requirement for py39 #37135'
```

4. Push to a named branch :

```
git push YOURFORK 1.1.x:auto-backport-of-pr-37144-on-1.1.x
```

5. Create a PR against branch 1.1.x, I would have named this PR:

> "Backport PR #37144 on branch 1.1.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to [suggest an improvement](https://github.com/MeeseeksBox/MeeseeksDev).
                