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] <support@github.com>

requirements_client.txt

@@ -1,5 +1,5 @@
 numpy==1.15.0
-PyYAML==5.1
+PyYAML==5.4
 requests==2.23
 msgpack-numpy==0.4.5
 pyzmq==19.0.0