- Edited CMakeLists.txt to search for Python with 3.6 as min version 3.6 is the
  Python version shipped by default with Ubuntu 18.04 LTS
- Add versions 3.6/3.7/3.8 to cmake_support/FindPython.cmake
- Prefer versioned Boost Python libraries and not first check for
  boost_python.so. In the example of Ubuntu 18.04, libboost_python.so
  is specific for Python 2 but libboost_python3.so is the one we want.
- apply the following command: 2to3-2.7 -n -w <QMEAN_DIR>
- resolve pickle encoding issue
  The linear local/global scorers are simply pickled to disk which can
  cause problems between Python 2 and 3. To avoid changing the code
  I simply loaded the scorer objects by explicitely defining the encoding
  and dumped them again => scorer = pickle.load(f_stream, encoding="latin1")
  The unit tests assure that the scorers still produce the same results.