Python 3.9 Support

[johnthagen]

Tracking issue for Python 3.9 support, set to release Oct 5, 2020.

Python 3.9rc1 has been released so it is possible to test in a near-final state. Travis CI python3.9-dev exists that could be used to test builds.

[skvark]

Different Python versions are provided by the manylinux images and multibuild and 3.9 is probably available also in them. Travis build host Python is not used in the builds or tests.

[johnthagen]

Python 3.9 released today.

[skvark]

Yeah, manylinux2014 Docker images in this repo need to be rebuilt when the base images have been updated from rc2 to the final version: https://github.com/pypa/manylinux/blob/master/docker/build_scripts/build_env.sh

[skvark]

I'll do the needed changes and rebuilds today, manylinux and multibuild were just updated to the 3.9 official release version.

Also we need to wait for numpy 1.19.3 release which should available in a week or so.

[skvark]

Waiting also for this: appveyor/ci#3541

Branch with the changes: https://github.com/skvark/opencv-python/tree/feat/python39

[skvark]

numpy/numpy#17482

[cclauss]

https://github.com/numpy/numpy/releases/tag/v1.19.3 Delivers numpy Python 3.9 binary wheels on all supported platforms.

[skvark]

Travis builds look ok, but I have to think what to do with Appveyor. Python 3.9 was installed only on the Visual Studio 2019 images (appveyor/ci#3541 (comment)) and that's not going to work for this project. Currently, Visual Studio 2015 is used for building the wheels. scikit-build does not recognize VS 2019 as a valid build tool so I'll have to install Python 3.9 manually to the Appveyor environment before builds. Upgrading to VS 2019 would also affect to the VC++ redistributable version needed to run these packages.

[skvark]

Release builds are now in progress but it will take over 24 hours before all wheels have been built and uploaded to PyPI.

[skvark]

Some of the wheels are not yet uploaded, opencv-python-headless hit the PyPI project size limit and prevents some builds from uploading new wheels. I'll delete some of the oldest releases from PyPI today to make room for new ones.