ShapelyDeprecationWarning for use of cascaded_union

[duckontheweb]

I get the following warning message when using polygon_geohasher.polygon_geohasher.geohashes_to_polygon:

.../python3.9/site-packages/polygon_geohasher/polygon_geohasher.py:56: ShapelyDeprecationWarning: The 'cascaded_union()' function is deprecated. Use 'unary_union()' instead.
    return cascaded_union([geohash_to_polygon(g) for g in geohashes])

I can suppress these using warnings.simplefilter but it might be good to switch to using unary_union as recommended in the warning message.

I'm happy to put in a PR for this.