Bump numpy from 1.16.5 to 1.18.1 (#188)

Bumps [numpy](https://github.com/numpy/numpy) from 1.16.5 to 1.18.1.
- [Release notes](https://github.com/numpy/numpy/releases)
- [Changelog](https://github.com/numpy/numpy/blob/master/doc/HOWTO_RELEASE.rst.txt)
- [Commits](https://github.com/numpy/numpy/compare/v1.16.5...v1.18.1

)

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