Update dependency pyyaml to v5

Author: dev-mend-for-github-com[bot]

requirements.txt

@@ -12,7 +12,7 @@ jinja2==2.10              # via aiohttp-jinja2
 markupsafe==1.1.0         # via jinja2
 multidict==4.5.2          # via aiohttp, yarl
 psycopg2==2.7.6.1         # via aiopg
-pyyaml==3.13
+pyyaml==5.4
 trafaret-config==2.0.2
 trafaret==1.2.0
 yarl==1.3.0               # via aiohttp