Bump numpy from 1.15.4 to 1.21.0

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] <support@github.com>

Author: dependabot[bot]

requirements.txt

@@ -1,7 +1,7 @@
 pyutilib==5.8.0
 jdcal==1.4
 Pyomo==5.5.1
-numpy==1.15.4
+numpy==1.21.0
 matplotlib==3.0.2
 scipy==1.1.0
 ipdb==0.11