I assume there was good reason to add it...

You might have to adapt CMake flags when building OpenStructure to variables specified here:
https://cmake.org/cmake/help/latest/module/FindPython.html

This code in cmake is currently not being used by any common build
we're using. I did therefore not test the changes but added a
comment to use that functionality with care.

This commit doesn't make OpenStructure work with Python 3. The goal of this
commit was to perform an automated port of the Python code and make it compile.
The performed steps:

- 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 version 3.6 to cmake_support/FindPython.cmake
- Adapt setup_boost macro in cmake_support/OST.cmake to prefer versioned
  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 <OST_DIR>
- adapt base/pymod/wrap_base.cc, gui/pymod/wrap_gui.cc and
  gui/pymod/export_message_widget.cc as PyString functionalities do not exist
  anymore in the Python 3 interpreter (replaced by PyUnicode)
- adapt gui/src/python_shell/python_interpreter_worker.hh to resolve issue
  discussed in https://stackoverflow.com/questions/23068700/embedding-python3-in-qt-5
  Long story short: Qt does a typedef for "slots" which causes trouble with
  other headers that are pulled in from the Python interpreter

Tested with Ubuntu 18.04 (in a vagrant box) following instructions in doc and
adding cmake flags "-DENABLE_STATIC=ON -DENABLE_INFO=OFF". Tested resulting
binaries "chemdict_tool" and "lddt" in Ubuntu-box and host-system (CentOS 7).

Horrible hack (execute_process) to work around the fact that the python modules in Ubuntu are now spread over many directories and symlinked everywhere

Substitutes DEBIAN_STYLE_LIBEXEC flag and manages location and name of lib and libexec folders on Debian/Ubuntu-based systems

fixed windows install statements to install dll instead of lib file

avoid additional Release and Debug directories
switched from deprrecated EXECUTABLE_OUTPUT_PATH to
CMAKE_RUNTIME_OUTPUT_DIRECTORY
deactivated unit test running for windows, as it is broken

changed python unit tests to run with python instead of ost

Remarks
-------

Python unit tests using blast, clustalw or the compound lib where
skipped on my system. There might be still some missing imports in
these tests.

fixed lib64/lib detection in copy dependencies script

cleaned up file list parsing for module and pymod macros

The idea was to speed up the build process by removing several
unneccessary targets and stage all headers in one go. However,
for some reason, CMake does not think that's a good idea. Only
parts of the headers get staged