bump py-yaml

requirements.txt

@@ -2,7 +2,7 @@ aiohttp>=3.6.0
 asyncio>=3.4.3
 netaddr>=0.7.19
 python-ldap>=3.0.0
-PyYAML~=5.1.2
+PyYAML~=5.4.1
 aiobotocore~=1.0.7
 botocore~=1.15.32
 aiofile~=1.5.2