Bump cython from 0.29.3 to 0.29.4 (#67)

Bumps [cython](https://github.com/cython/cython) from 0.29.3 to 0.29.4.
- [Release notes](https://github.com/cython/cython/releases)
- [Changelog](https://github.com/cython/cython/blob/master/CHANGES.rst)
- [Commits](https://github.com/cython/cython/compare/0.29.3...0.29.4

)

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