allow ellipsis

[zedzhen]

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Closes #311

[dataflake]

Ellipsis seems to have been removed for a reason. Have you looked at 61d26b3 and the referred https://bugs.python.org/issue36917#msg342583?

[zedzhen]

I looked. It was allowed at #150 because they didn't know how to disallowed it. In #171, it was disallowed because they found a good way to disallow it back. There were no specific reasons for the disallow.

[dataflake]

After reading up on the Ellipsis type, which is just a constant, this should be safe.