Support Python 3.9

[anthrotype]

The stable release of Python 3.9 was released last week.
We need to build wheels for 3.9 as well.
I think it should be enough to simply add 3.9 to the build matrix of both Travis and Appveyor, after having updated the multibuild submodule to the latest "devel" branch which should support 3.9 now.
We could also drop support for 3.5 since it has reached EOL.

[anthrotype]

Looks like Appveyor still hasn't added support for Python 3.9
appveyor/ci#3541

[ajkaijanaho]

Looks like Appveyor still hasn't added support for Python 3.9
appveyor/ci#3541

That ticket is closed by now. Any updates?

[anthrotype]

Thanks for the reminder. The python3.9 wheels for Windows are currently missing from PyPI https://pypi.org/project/Brotli/1.0.9/#files

I have just pushed an empty commit to trigger a new build on Appveyor, if that passes (as it should now) I will upload the generated wheels as well. Stay tuned.

[anthrotype]

Appveyor python3.9 build passed successfully:
https://ci.appveyor.com/project/szabadka/brotli-wheels/builds/37118320

The files Brotli-1.0.9-cp39-cp39-win32.whl and Brotli-1.0.9-cp39-cp39-win_amd64.whl have been uploaded to https://pypi.org/project/Brotli/1.0.9/#files