Bug#972566: python3-opencv: build python extensions for all supported python versions

[Drew Parsons]

Package: python3-opencv
Version: 4.2.0+dfsg-6+b4
Severity: normal

Currently python3-opencv only builds python extensions
(/usr/lib/python3/dist-packages/cv2.cpython-38-x86_64-linux-gnu.so)
for the default version of python (python3.8 at the moment).

It would be nice if the extension module could be built for all
supported pythons (python3.9 is now in the archive).

Currently opencv Build-Depends: python3-dev, hence only builds against
the default python.

Will the other python versions be built automatically if that is updated to
 Build-Depends: python3-all-dev   ?