Bump numpy from 1.15.4 to 1.21.0 in /docs

Bumps [numpy](https://github.com/numpy/numpy) from 1.15.4 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](numpy/numpy@v1.15.4...v1.21.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

docs/requirements.txt

@@ -38,7 +38,7 @@ nbconvert==5.3.1
 nbformat==4.4.0
 nbsphinx==0.3.5
 notebook==6.1.5
-numpy==1.15.4
+numpy==1.21.0
 optlang==1.4.3
 packaging==18.0
 pandas==0.23.4