Bump pyyaml from 3.13 to 5.1

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.13 to 5.1.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@3.13...5.1)

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

requirements.txt

@@ -77,7 +77,7 @@ pytesseract==0.2.5
 pytest==4.4.1
 python-dateutil==2.7.3
 pytz==2018.5
-PyYAML==3.13
+PyYAML==5.1
 pyzmq==17.1.2
 qtconsole==4.4.3
 requests==2.21.0