Prefers yaml.safe_load in config test conftest
Prefers yaml.safe_load in config test conftest

As a result of @garrettr's review on #1859, amending the YAML file
reading to use the `safe_import` functionality, which evidently should
be preferred pretty much everywhere [0].

Also updated the logic to shove the `open` call into a `with` block, as
a standard Python best practice.

[0] https://security.openstack.org/guidelines/dg_avoid-dangerous-input-parsing-libraries.html

conorsch/securedrop,ageis/securedrop,garrettr/securedrop,micahflee/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,micahflee/securedrop,ageis/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,ageis/securedrop,ageis/securedrop,micahflee/securedrop,heartsucker/securedrop,garrettr/securedrop,conorsch/securedrop,heartsucker/securedrop,conorsch/securedrop,conorsch/securedrop,garrettr/securedrop,ehartsuyker/securedrop,garrettr/securedrop,micahflee/securedrop,heartsucker/securedrop,conorsch/securedrop