Merge pull request #9293 from ARMmbed/cmonr-pyyaml-fix

Updated pyyaml due to possible vulnerability

Author: Cruz Monrreal

requirements.txt

@@ -4,7 +4,7 @@ prettytable==0.7.2
 Jinja2>=2.7.3,<=2.10
 intelhex>=1.3,<=2.2.1
 junit-xml==1.8
-pyYAML==3.13
+pyyaml==4.2b1
 urllib3[secure]==1.23
 requests>=2.20,<2.21
 intervaltree>=2,<3