Bump numpy from 1.19.5 to 1.21.0 in /doc

Bumps [numpy](https://github.com/numpy/numpy) from 1.19.5 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](numpy/numpy@v1.19.5...v1.21.0)

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

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

doc/requirements.txt

@@ -17,7 +17,7 @@ Jinja2==2.11.3
 kiwisolver==1.3.1
 MarkupSafe==1.1.1
 matplotlib==3.3.4
-numpy==1.19.5
+numpy==1.21.0
 numpydoc==1.1.0
 packaging==20.9
 parso==0.8.2