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.

docs/installation.rst

@@ -42,7 +42,6 @@ to resolve the following dependencies before the pip installer is run:
     * geopandas
     * numpy
     * pyproj >=2.1.0
-    * scikit-image
     * shapely