Bump pyyaml from 5.1 to 5.3

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 5.1 to 5.3.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@5.1...5.3)

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

Author: dependabot[bot]

requirements.txt

@@ -25,7 +25,7 @@ pygobject==3.26.1
 pykerberos==1.1.14
 pyOpenSSL==17.5.0
 pyxdg==0.26
-PyYAML==5.1
+PyYAML==5.3
 requests==2.20.4
 SecretStorage==2.3.1
 simplejson==3.13.2