Bump numpy from 1.17.1 to 1.21.0 in /airflow_docker

Bumps [numpy](https://github.com/numpy/numpy) from 1.17.1 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](https://github.com/numpy/numpy/compare/v1.17.1...v1.21.0)

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

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

airflow_docker/requirements.txt

@@ -1,7 +1,7 @@
 kafka-python==1.4.6
 Keras==2.2.5
 mlflow==1.23.1
-numpy==1.17.1
+numpy==1.21.0
 pandas==0.25.1
 scikit-learn==0.21.3
 scipy==1.3.1