Bump pyyaml from 3.11 to 5.1

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.11 to 5.1.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@3.11...5.1)

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

requirements.txt

@@ -1,4 +1,4 @@
-PyYAML==3.11
+PyYAML==5.1
 argparse==1.2.1
 coverage==3.7.1
 pep8==1.5.7