Fix PyYAML version on requirements-step-2.txt

Author: laliux

app/requirements-step-2.txt

@@ -12,7 +12,7 @@ python-telegram-bot>=10.0.1
 webcord>=0.2
 jinja2>=2.10
 requests>=2.20.0
-PyYAML>=3.12
+PyYAML>=5.1
 tulipy>=0.2.1
 matplotlib>=3.0.1
 scipy>=1.1.0