Add python3 subpackage (bz# 1482979).
    
    - For Fedora releases, add a python3 subpackage.
    - Disable the LEMON compile due to a bug in lemon packaging which
      prevents vigra from successfully finding the library (bz# 1526647).
    - Move python shebang fix from a patch file into the spec file to easily
      choose between python 2/3 based on the release being built.
    - Remove unused patches.
    - Remove legacy _python_sitearch as all supported release now have this
      defined.
    - Add conditions for boost-python to allow compiling on releases <= f27.
    - Fix the vigra-config docdir to point to the packaged location.