title: Unable to build opencv-python with python 3.8
body: <p>OS:ubuntu
  I am able to install opencv-python from the command line. However,when I tried to install my self-built package with opencv-python dependencies, it failed to biuld:</p>
  <pre><code>Building wheels for collected packages: opencv-python
    Building wheel for opencv-python (PEP 517): started
    Building wheel for opencv-python (PEP 517): finished with status 'error'
    ERROR: Complete output from command /usr/local/bin/python /usr/local/lib/python3.6/site-packages/pip/_vendor/pep517/_in_process.py build_wheel /tmp/tmp7jntqv2r:
    ERROR: Not searching for unused variables given on the command line.
    CMake Error: CMake was unable to find a build program corresponding to &quot;Ninja&quot;.  CMAKE_MAKE_PROGRAM is not set.  You probably need to select a different build tool.
    -- Configuring incomplete, errors occurred!
    See also &quot;/tmp/pip-install-utbicth7/opencv-python/_cmake_test_compile/build/CMakeFiles/CMakeOutput.log&quot;.
    Not searching for unused variables given on the command line.
    CMake Error: CMake was unable to find a build program corresponding to &quot;Unix Makefiles&quot;.  CMAKE_MAKE_PROGRAM is not set.  You probably need to select a different build tool.
    -- Configuring incomplete, errors occurred!
    See also &quot;/tmp/pip-install-utbicth7/opencv-python/_cmake_test_compile/build/CMakeFiles/CMakeOutput.log&quot;.
  
  
  -- Trying &quot;Unix Makefiles&quot; generator - failure
    --------------------------------------------------------------------------------
    
    ********************************************************************************
    scikit-build could not get a working generator for your system. Aborting build.
    
    Building Linux wheels for Python 3.6 requires a compiler (e.g gcc).
    It can be installed using debian package manager:
    
      sudo apt-get install build-essential
    
    To build compliant wheels, consider using the manylinux system described in PEP-513.
    Get it with &quot;dockcross/manylinux-x64&quot; docker image:
    
      https://github.com/dockcross/dockcross#readme
    
    For more details, please refer to scikit-build documentation:
    
      http://scikit-build.readthedocs.io/en/latest/generators.html#linux
    
    ********************************************************************************
    ----------------------------------------
    ERROR: Failed building wheel for opencv-python
  
  </code></pre>
  <p>I suspect issue is with scikit-build. I checked I have ninja, make/gcc all available. I set manually CMAKE_MAKE_PROGRAM=/usr/bin/make.</p>
  <p>But still failed with same error. I</p>