initial Python 3 commit

- 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 <PROMOD3_DIR>
- replace commands added in cmake to build documentation from
  sphinx-build <args> to python_executable -m sphinx <args>
  The reason is that sphinx-build is a short shell script starting with a
  shebang #!python. In Debian based systems this means by default Python 2
  if you're not running a virtual env or manually changed that behaviour.
  PROBLEM: cmake is NOT looking for the presence of Sphinx anymore and you'll
  get errors at build time if its missing. A cmake macro that checks whether
  a certain module can be imported would be nice.
- replace r.__next__ with r.next to get the next ResidueHandle
- edit unit tests that expected strict string output (pm3_argparse) that
  might change depending on how json generates strings of dictionaries