Bump pyyaml from 5.3 to 5.4

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 5.3 to 5.4.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@5.3...5.4)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements.txt

@@ -51,7 +51,7 @@ pytorch-pretrained-bert==0.6.2
 pytorch-transformers==1.2.0
 pytz==2019.3
 pyuca==1.2
-PyYAML==5.3
+PyYAML==5.4
 regex==2020.2.20
 requests==2.23.0
 requests-oauthlib==1.3.0