Upgrade Python to 3.7.3 and add PyYAML.

We should try and track mainline Python as much as possible.

PyYAML was a sub-dependency of a dev dependency but was being included
in the translations utility. Bundling only the production Python
dependencies was not working because of this.

Pipfile

@@ -22,6 +22,7 @@ apache-libcloud = "*"
 lockfile = "*"
 "flask-rq2" = "*"
 werkzeug = "~= 0.14.1"
+PyYAML = "*"
 
 [dev-packages]
 bandit = "*"
@@ -41,7 +42,7 @@ blinker = "*"
 pytest-mock = "*"
 
 [requires]
-python_version = "3.6.6"
+python_version = "3.7.3"
 
 [pipenv]
 allow_prereleases = true