Bump pyyaml from 3.13 to 5.4

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.13 to 5.4.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](https://github.com/yaml/pyyaml/compare/3.13...5.4)

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

requirements.txt

@@ -33,7 +33,7 @@ pyparsing==2.3.0
 python-dateutil==2.7.5
 pytz==2018.5
 PyWavelets==1.0.1
-PyYAML==3.13
+PyYAML==5.4
 requests==2.20.1
 scikit-image==0.14.1
 scikit-learn==0.20.0