Bump pyyaml from 5.1.2 to 5.4

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 5.1.2 to 5.4.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@5.1.2...5.4)

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

requirements.txt

@@ -18,7 +18,7 @@ pyparsing==2.4.2
 pypng==0.0.20
 python-dateutil==2.8.0
 PyWavelets==1.0.3
-PyYAML==5.1.2
+PyYAML==5.4
 scikit-image==0.15.0
 scikit-learn==0.21.3
 scipy==1.3.1