Make PyIlmBase compatible with Python 3.x

[sybrenstuvel]

With Python 2 being phased out in the near future (within 3 years), it doesn't make sense for current software to be aiming only at Python 2. PyIlmBase should build on Python 3.x.

PYTHON=/usr/bin/python3.5 ./configure --with-boost-python-libname=libboost_python-py35 doesn't work on Ubuntu 16.10, and fails with this error:

...
checking for python version... 3.5
checking for python platform... linux
checking for python script directory... ${prefix}/lib/python3.5/site-packages
checking for python extension module directory... ${exec_prefix}/lib/python3.5/site-packages
  File "<string>", line 1
    from distutils.sysconfig import *; print get_python_inc()
                                                          ^
SyntaxError: invalid syntax
  File "<string>", line 1
    from distutils.sysconfig import *; print get_config_var("LIBDIR")
                                                          ^
SyntaxError: invalid syntax
checking for boost::python... no
*** Could not run the boost::python test program, checking why...
*** The test program could not be compiled.  Is boost::python installed?
*** Check that the cflags (below) includes the boost::python include directory
***
*** Flags used by the test:
***     cflags: -g -O2  -I -g -O2  -I 
***     ldflags:    -L    -L
***
*** You can also run configure with --disable-boostpythontest to skip this test.
configure: error: Could not compile boost::python test program.

Using print statements is not supported in Python 3 any more. Python 2 (2.6+) can even be told to use print() functions by including from __future__ import print_function at the top of the Python file.