Bump pyyaml from 3.11 to 5.4

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.11 to 5.4.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@3.11...5.4)

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

Author: dependabot[bot]

requirements.txt

@@ -5,7 +5,7 @@ Flask-Mail==0.9.1
 Jinja2==2.11.3
 MarkupSafe==0.23
 Pillow==2.8.1
-PyYAML==3.11
+PyYAML==5.4
 Werkzeug==0.15.5
 argparse==1.2.1
 astLib==0.8.0