Speed up polygonizer, drop Python 2.7 and update requirements.
- Added test and test data for geo.raster.conversion.raster2polygon().
- Replaced skimage downsampling by GDAL variant.
- Removed scikit-image dependency.
- Dropped Python 2.7 support due to end-of-life status.
- Dropped compatibility module since this was Python 2.7 specific.
- Dropped requirements six and packaging.
- Added some docs/.rst files to .gitignore.
- Updated minimal version of pyproj to 2.5.0.