Mark slow tests, update `__eq__` methods, pin reqs

[brl0]

Fixes failing tests after updating to pandas 1.2.1. Mark slow tests and add option --skip-slow.

[brl0]

I realized after I first created this PR that changes had already been merged to fix the issues with the pandas update. To summarize other changes that I think are useful or necessary:

• Add --skip-slow option (slow tests are still run by default)
• Mark various tests as slow based on run time as captured by pytest --durations=0
• Pin numpy<1.20 due to test failures on all platforms
• Pin pyarrow<2 due to test failures on MacOS
• Add feature to Geometry class __eq__ method that if other object is a container, the equality method on the container is called, so now performing an equality check between a geometry object and a geometry array should return correct results, which should be a bool array, whereas previously it would simply return False because the objects were not the same immediate type.
• Add feature to GeometryArray class __eq__ method to allow comparison of an individual element to all objects in the array, returning an array of bool.
• Add NotImplementedError for __contains__ method.
• Sort imports on modified files

Let me know if there are any questions or concerns about any of these changes.

[brl0]

Covered by another PR, closing.