AppVeyor: Test on Python 3.9 final

[hugovk]

Fixes #4953.

Changes proposed in this pull request:

• AppVeyor now supports Python 3.9 final
• It requires the Visual Studio 2019 image (also works for 3.6)
• And the setuptools bump
• Python 3.9 appveyor/ci#3541

(It's like buses. You wait for ages and then two come along at once.)

[nulano]

(It's like buses. You wait for ages and then two come along at once.)

😄

It requires the Visual Studio 2019 image (also works for 3.6)

Hmm... The idea for using VS2017 was to test the build script against multiple versions of VS.
Maybe wait a bit longer to see if they are planning to add it to the other images appveyor/ci#3541 (comment)?

I currently only have VS2017 installed on my system and it is what I use to compile Pillow, but that doesn't give immediate test results when updating one of the dependencies (to see if the patches need to be updated), especially since I don't always rebuild dependencies when changing branches.

[hugovk]

Sure, let's wait, we're in no rush to update this, we already have GHA testing 3.9.

If AppVeyor aren't going to put 3.9 on VS2017, maybe 3.9/VS2019 and 3.6/VS2017 is possible?

[nulano]

If AppVeyor aren't going to put 3.9 on VS2017, maybe 3.9/VS2019 and 3.6/VS2017 is possible?

Looks like it is not too difficult: https://www.appveyor.com/docs/build-environment/#using-multiple-images-for-the-same-build

[hugovk]

Updated!

[nulano]

And it works!

Python 3.6 x64 on VS2017: https://ci.appveyor.com/project/Python-pillow/pillow/builds/35991212/job/nvmoe990w4voffvq?fullLog=true#L129
Python 3.9 32-bit on VS2019: https://ci.appveyor.com/project/Python-pillow/pillow/builds/35991212/job/oeslqec5uu3ekov7?fullLog=true#L129

[hugovk]

(11 hours later and Travis CI still hasn't finished...)