Bump tensorflow from 1.15.4 to 2.4.0 in /testing/scripts

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 1.15.4 to 2.4.0.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v1.15.4...v2.4.0)

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

testing/scripts/dev_requirements.txt

@@ -11,7 +11,7 @@ nbconvert==5.6.1
 alibi==0.5.2
 shap==0.35.0
 matplotlib==3.1.3
-tensorflow==1.15.4
+tensorflow==2.4.0
 
 # 2nd lvl dep on cov required to avoid sqllite dep
 coverage==4.5.4