Bump numpy from 1.16.4 to 1.21.0 in /examples/water_boiler

Bumps [numpy](https://github.com/numpy/numpy) from 1.16.4 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@v1.16.4...v1.21.0)

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

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

Author: dependabot[bot]

examples/water_boiler/requirements.txt

@@ -1,7 +1,7 @@
 cycler==0.10.0
 kiwisolver==1.1.0
 matplotlib==3.1.0
-numpy==1.16.4
+numpy==1.21.0
 pyparsing==2.4.0
 python-dateutil==2.8.0
 simple-pid==1.0.1