Fix shapely deprecation warnings

[ianthomas23]

Running the tests results in a large number of shapely deprecations warnings of these types:

ShapelyDeprecationWarning: __getitem__ for multi-part geometries is deprecated and will be removed in Shapely 2.0. Use the `geoms` property to access the constituent parts of a multi-part geometry.
ShapelyDeprecationWarning: __len__ for multi-part geometries is deprecated and will be removed in Shapely 2.0. Check the length of the `geoms` property instead to get the  number of parts of a multi-part geometry.
ShapelyDeprecationWarning: Accessing the 'ctypes' attribute is deprecated, and will not be possible any more in Shapely 2.0
ShapelyDeprecationWarning: Iteration over multi-part geometries is deprecated and will be removed in Shapely 2.0. Use the `geoms` property to access the constituent parts of a multi-part geometry.
ShapelyDeprecationWarning: The array interface is deprecated and will no longer work in Shapely 2.0. Convert the '.coords' to a numpy array instead.
ShapelyDeprecationWarning: The 'cascaded_union()' function is deprecated. Use 'unary_union()' instead.

This PR follows the advice contained in those warnings so that they no longer occur.

I still see 3 test failures but they are all parquet-related, presumably issue #84.

[ianthomas23]

This needs an existing maintainer to approve the CI.

[jbednar]

Looks great! Do you know if these changes require putting a lower bound on the shapely version? I.e., were they always valid and are now being preferred, or do these changes rely on changes only introduced relatively recently and thus need to be reflected in the setup.py constraints?

[jorisvandenbossche]

Those APIs have always existed, so a lower bound should (AFAIK) not be needed.

From a quick look, seems all good!

[ianthomas23]

I've checked the test failures and they are indeed the 3 parquet-related failures. There is also this on Python 3.6:

INTERNALERROR> Hypothesis 6.35.1 requires Python 3.7 or later.

I presume that support for Python 3.6 will be dropped soon? I note that numpy have already dropped support for 3.7.

[ianthomas23]

Merging this as it successfully removes the Shapely warnings.