Upgrade to PyYaml 5.1 (Sceptre#681)

Updates the PyYaml version to 5.1.

Adds in YamlLoader as per yaml/pyyaml#292

Other incompatible changes were reviewed
(yaml/pyyaml#265) and the yaml.Loader appears
to the be only concern for now.

[Resolves Sceptre#665]

requirements.txt

@@ -15,7 +15,7 @@ packaging==16.8
 pygments==2.2.0
 pytest-runner>=3.0.0,<3.1.0
 pytest>=3.2.0,<3.3.0
-PyYaml>=4.2b1,<5.0
+PyYaml>=5.1,<6.0
 readme-renderer>=24.0
 setuptools>=40.6.2
 six>=1.11.0,<2.0.0

sceptre/cli/helpers.py

@@ -112,19 +112,21 @@ def _generate_yaml(stream):
             try:
                 if isinstance(item, dict):
                     items.append(
-                            yaml.safe_dump(item, default_flow_style=False, explicit_start=True)
+                        yaml.safe_dump(item, default_flow_style=False, explicit_start=True)
                     )
                 else:
                     items.append(
-                            yaml.safe_dump(
-                                yaml.load(item), default_flow_style=False, explicit_start=True)
+                        yaml.safe_dump(
+                            yaml.load(item, Loader=yaml.FullLoader),
+                            default_flow_style=False, explicit_start=True
+                        )
                     )
             except Exception:
                 print("An error occured whilst writing the YAML object.")
         return yaml.safe_dump(
-                    [yaml.load(item) for item in items],
-                    default_flow_style=False, explicit_start=True
-                )
+            [yaml.load(item, Loader=yaml.FullLoader) for item in items],
+            default_flow_style=False, explicit_start=True
+        )
     else:
         try:
             return yaml.safe_loads(stream)

setup.py

@@ -13,7 +13,7 @@
 install_requirements = [
     "boto3>=1.3,<2.0",
     "click==7.0",
-    "PyYaml>=4.2b1,<5.0",
+    "PyYaml>=5.1,<6.0",
     "Jinja2>=2.8,<3",
     "packaging==16.8",
     "colorama==0.3.9",