Bump pyyaml from 3.12 to 5.1 (#5)

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.12 to 5.1.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@3.12...5.1)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements.txt

@@ -1,3 +1,3 @@
 pytest==3.4.0
-PyYAML==3.12
+PyYAML==5.1
 wheel