thumbor-9999.ebuild: Hard-code OpenCV Python version

thumbor-9999.ebuild: Hard-code OpenCV Python version

media-libs/opencv uses python-single-r1, which means it doesn't like
our ${PYTHON_USEDEP}:

  emerge: there are no ebuilds built with USE flags to satisfy
  "media-libs/opencv[python_targets_python2_7(-)?,-python_single_target_python2_7(-)]".

Note the '-' in front of python_single_target_python2_7.  I'm not sure
what the best way to handle this is, since *without* the OpenCV
dependency, Thumbor could theoretically be installed for multiple
Python versions simultaneously.  It would be nice to translate the
OpenCV ebuild to python-r1 or distutils-r1, but it ties the Python
binding config into its main CMake setup:

  local mycmakeargs=(
    ...
    $(cmake-utils_use_build python opencv_python)
    ...
  )

That makes it hard to step in and build Python bindings for a
different version.  Until someone thinks up (or tells me about) a
cleaner way to handle this, I'm just going to hardcode the
single-version we need.