Check that PyPI supports manylinux wheels

[skvark]

Just a minor check.

[c-martinez]

This note seems to imply that PyPI does not support platform wheels. But more recently, PEP513 mentions PyPI support for manylinux wheels.

More importantly, there are packages in PyPI (such as Pillow) which already provide manylinux wheels. So I think we are ok.

Also, the way the Pillow wheels are built seems interesting.

[skvark]

Yes, it seems that manylinux wheels can be added to PyPi.

The OS X builds could be done as they are done in the Pillow repo. I think also that we could use the same submodule method for obtaining OpenCV. That way the version could be locked to certain commit and updated manually whenever needed.