Bump numpy from 1.19.2 to 1.19.4 (#430)

Bumps [numpy](https://github.com/numpy/numpy) from 1.19.2 to 1.19.4.
- [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/v1.19.2...v1.19.4)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements.txt

@@ -1,4 +1,4 @@
-numpy == 1.19.2
+numpy == 1.19.4
 scipy == 1.5.3
 pandas == 1.1.3
 matplotlib == 3.3.2