pip install opencv takes 24 minutes on Python 3.9 on Travis CI

[cclauss]

.travis.yml

language: python
python: 3.9
install: pip install opencv-python  # This step takes 1,464 seconds
script: true

[mshabunin]

I believe this happens because there are no prebuilt opencv-python wheels for python 3.9 yet (https://pypi.org/project/opencv-python/#files) and the package is built from sources.

Anyway, this is out of scope of this issue tracker. See opencv/opencv-python#382