Unit tests successfully run through with Python 3.9 on Fedora 33 (no GUI)

This can happen if python has been compiled with a specialized
malloc.

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

Fixed issue in FindPython: requested Python version was only tested to be bigger than requirement, not equal to

fixed windows install statements to install dll instead of lib file

On many systems, there are several parallel python
installations. There is usually a "python" symlink that
points to the currently active python version. That might
not be the one we need. Instead of picking up that one, we
now search for python$VERSION and as a last resort use
python.

Also, make sure found python lib/binaries match. This
fixes BZDNG-388.

On many systems, there are several parallel python
installations. There is usually a "python" symlink that
points to the currently active python version. That might
not be the one we need. Instead of picking up that one, we
now search for python$VERSION and as a last resort use
python.

This reverts commit 77521f73.

changed cmake scripts to allow building as subproject / moved common configuration from startup scripts to separate script

git-svn-id: https://dng.biozentrum.unibas.ch/svn/openstructure/trunk@1742 5a81b35b-ba03-0410-adc8-b2c5c5119f08