Upgraded from v0.4 to v0.9 - EllipseSkyRegion no longer hashable?

Hi!

First off, thanks for making this module, it has been very useful in my own software. I've been very bad at keeping dependencies updated, and have recently started updating them all. Testing highlighted a problem and throws an error:

TypeError: unhashable type: 'EllipseSkyRegion'

I'm wondering if this was a deliberate behaviour change?

I was essentially using an intersection between two sets to determine which region instances where present in both, which I should be able to achieve in other ways (I believe the region class has an __eq__ method implemented), but using the sets and intersection was neat.

Start by reproducing the TypeError when an EllipseSkyRegion is placed in a set, then compare the behavior between versions v0.4 and v0.9. Inspect the region class's __eq__ implementation and related hash behavior. Done means establishing whether the change was intentional and, if not, restoring or documenting compatible set-intersection behavior.

Written by the indexing model from the issue text.