This reverts commit ef7bfdb6.

Didn't work with Sphinx 1.6.7 which needs an extra option at the start.
This needs to be fixed cleanly, or we need to upgrade the Sphinx version
requirements

This likely requires an up-to-date sphinx (tested with current version, 2.3.0)

- os.path.walk does not exist in Python3 anymore, use os.walk instead
  (changed interface)
- sphinx-build tried really hard to use Python 2. Import sphinx from
  current interpreter instead and directly call its __main__ function

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

Copied change from PM3 (commit d16162baffb140de971e24aad71c8cce257a6fce).

Instead of creating a lib/openstructure subdirectory containing
all ost modules, use lib/python$MAJOR.$MINOR/site-packages. This
follows more closely what other projects are doing. The main
motivation, however is to simplify deploment of tools built on
OpenStructure.

git-svn-id: https://dng.biozentrum.unibas.ch/svn/openstructure/trunk@2461 5a81b35b-ba03-0410-adc8-b2c5c5119f08

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Look first for sphinx-build, then for sphinx-build-2.6. As part
of this change, it is now required to run make.py with ost as
we make use of the settings.Locate() function

git-svn-id: https://dng.biozentrum.unibas.ch/svn/openstructure/trunk@2390 5a81b35b-ba03-0410-adc8-b2c5c5119f08

git-svn-id: https://dng.biozentrum.unibas.ch/svn/openstructure/trunk@2384 5a81b35b-ba03-0410-adc8-b2c5c5119f08

Usage:
  ost doc/to_sphinx.py ost.mol.EntityHandle

git-svn-id: https://dng.biozentrum.unibas.ch/svn/openstructure/trunk@2382 5a81b35b-ba03-0410-adc8-b2c5c5119f08