Bump pyyaml from 3.13 to 5.4

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.13 to 5.4.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@3.13...5.4)

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

Author: dependabot[bot]

requirements.txt

@@ -83,7 +83,7 @@ python-dateutil==2.7.2
 python-jsonrpc-server==0.3.2
 python-language-server==0.25.0
 pytz==2018.3
-PyYAML==3.13
+PyYAML==5.4
 pyzmq==18.1.1
 requests==2.20.0
 rope==0.14.0