Fixed py38 upgrade issue on macOS on Travis
Fixed py38 upgrade issue on macOS on Travis

Details:

* The Travis runs on macOS used the default OS image (osx_image=xcode9.4), where
  the python3 command is Python 3.7.7. The installation of libxml2 requires
  Python 3.8 and installs that. That installation does not work properly
  and fails running Pip.

  Fixed that by specifying a higher version of the OS image for Python 3,
  by specifying:
    osx_image: xcode12
  This also has Python 3.7.7 behind the python3 command, but it successfully
  upgrades that to 3.8.

Signed-off-by: Andreas Maier <0903150dae73adcf31eccfd2633bf70bbc40bd52@gmx.de>

pywbem/pywbemtools,pywbem/pywbemtools