Update PyYAML version requirement to >=5.4

To alleviate CVEs in older versions identified by GitHub Dependabot.

Author: penyuan

requirements.txt

@@ -7,5 +7,5 @@ dash-bootstrap-components>=0.11.1
 numpy~=1.17.3
 pandas~=0.25.2
 plotly>=4.10.0 # To solve: https://github.com/plotly/plotly.py/issues/1630
-PyYAML~=5.1.2
+PyYAML>=5.4
 requests~=2.22.0