Fixed building against sip-4.19 (thanks to Sandro Mani for the patch).
    
    Updated and modernized spec file:
      - Simplified filtering of auto-generated provides.
      - Replaced %{python_*} macros with %{python2_*} macros.
      - Removed %qwt_dep and replaced it with a fixed qwt dependency.
      - Removed trailing white-space.