Fix FTBFS with recent python-defaults (#968841)

• Merge Matthias Klose's NMU, from dgit

• Change shebang on Python 2 scripts to #!/usr/bin/python2

  /usr/bin/python is no longer an indirect build dependency, but python2 now is.

  Similarly, when invoking setup.py from the Makefile, use python2.

  Closes: #968841

• Use non-deprecated syntax for python-librdf dependency

  The comparator ">" is deprecated because it looks as though it ought to mean "strictly greater", but actually means "greater than or equal". The version of python-librdf that stops hard-coding python (as opposed to python2) is 1.0.17.1+dfsg-1.4, so we do want greater than or equal, ">=", rather than strictly greater, ">>".

• Update changelog

• Document the python-librdf change in the previous version