Fix pyyaml deprecated warning

We were getting the following warning with the latest version of pyyaml:

    YAMLLoadWarning: calling yaml.load() without Loader=... is
    deprecated, as the default Loader is unsafe. Please read
    https://msg.pyyaml.org/load for full details.

requirements/base.txt

@@ -26,7 +26,7 @@ pyasn1==0.4.5             # via pyasn1-modules, rsa
 pycparser==2.19           # via cffi
 pyjwt==1.7.1              # via adal
 python-dateutil==2.8.0    # via adal, kubernetes
-pyyaml==4.2b4
+pyyaml==5.1
 requests-oauthlib==1.2.0  # via kubernetes
 requests==2.21.0          # via adal, kubernetes, requests-oauthlib
 rsa==4.0                  # via google-auth

requirements/dev.txt

@@ -33,7 +33,7 @@ pycparser==2.19
 pyinstaller==3.4
 pyjwt==1.7.1
 python-dateutil==2.8.0
-pyyaml==4.2b4
+pyyaml==5.1
 requests-oauthlib==1.2.0
 requests==2.21.0
 rsa==4.0

tutor/serialize.py

@@ -1,7 +1,7 @@
 import yaml
 
 def load(stream):
-    return yaml.load(stream)
+    return yaml.load(stream, Loader=yaml.SafeLoader)
 
 def dump(content, fileobj):
     yaml.dump(content, fileobj, default_flow_style=False)