Coverage remained the same at 77.369% when pulling 27120ac on jorisvandenbossche:refactor-2.0-linearring-pickle into 6821fb8 on Toblerity:shapely-2.0.

The windows failure is unrelated (it's also a pickling error, but already existing and related to encoding)

I am not too familiar with the internal details of pickling, but I know we use __reduce__ in several other packages I contribute to as well.
Typically, I think __setstate__ can be used to update the "state" (__dict__) of a python object after it has been initialized. But specifically in this case, the pygeos Geometry's are now Python extension types that don't have a mutable state (like builtins, you also can't assign random attributes to it). So I think in this case we actually have to use __reduce__. For example, the underlying GEOSGeometry pointer (which would need to be changed from LineString to LinearRing) is stored in _ptr attribute, but that's a readonly attribute and can't be changed from Python (only in C).

Going to merge this, so I can update #1163