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>

Author: dependabot[bot]

requirements_dev.txt

@@ -7,5 +7,5 @@ tox==2.3.1
 coverage==4.1
 Sphinx==1.4.4
 cryptography==3.2
-PyYAML==5.1
+PyYAML==5.4
 pytest==2.9.2