Implement `__sizeof__`

Joseph Weston requested to merge jbweston/tinyarray:feature/sizeof into master

Implement the __sizeof__ member function to get the size in bytes of the full Python object. This fixes #3 (closed).

I have also added the Gitlab-CI config to the Git repository so that we can have automated testing for Tinyarray as for Kwant. I can pull this out into a separate merge request if needed.

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