Bump numpy from 1.18.5 to 1.21.0 in /docs/requirements (#57)

Bumps [numpy](https://github.com/numpy/numpy) from 1.18.5 to 1.21.0.
- [Release notes](https://github.com/numpy/numpy/releases)
- [Changelog](https://github.com/numpy/numpy/blob/main/doc/HOWTO_RELEASE.rst.txt)
- [Commits](https://github.com/numpy/numpy/compare/v1.18.5...v1.21.0)

---
updated-dependencies:
- dependency-name: numpy
  dependency-type: direct:production
...

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