Bump pyyaml from 5.1 to 5.4

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 5.1 to 5.4.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@5.1...5.4)

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

Author: dependabot[bot]

requirements.txt

@@ -23,7 +23,7 @@ pyparsing==2.4.0
 PyQt5==5.12.1
 PyQt5-sip==4.19.15
 python-dateutil==2.8.0
-PyYAML==5.1
+PyYAML==5.4
 scikit-learn==0.20.3
 scipy==1.2.1
 six==1.12.0