Bump pyyaml from 5.1.2 to 5.4

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 5.1.2 to 5.4.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@5.1.2...5.4)

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

requirements.txt

@@ -3,7 +3,7 @@ conllu==1.3.1
 doit==0.32.0
 html5lib==1.0.1
 pydash==4.7.4
-pyyaml==5.1.2
+pyyaml==5.4
 quart==0.6.15
 requests==2.21.0
 yattag==1.13.2