Bump numpy from 1.18.0 to 1.19.4

Bumps [numpy](https://github.com/numpy/numpy) from 1.18.0 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](numpy/numpy@v1.18.0...v1.19.4)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements.txt

@@ -1,4 +1,4 @@
-numpy==1.18.0
+numpy==1.19.4
 pandas>=0.25.1
 scipy>=1.5.2
 histogrammar>=1.0.12