Bump pyyaml from 3.12 to 5.4

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.12 to 5.4.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@3.12...5.4)

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

requirements.txt

@@ -13,7 +13,7 @@ py==1.5.3
 pylibmc==1.5.2
 PySocks==1.6.8
 python-dateutil==2.4.0
-PyYAML==3.12
+PyYAML==5.4
 requests==2.20.1
 requests-oauthlib==0.8.0
 six==1.11.0