Uses deprecated yaml.load()

[mbanck]

Describe the bug
From Debian Bug 1022080 (https://bugs.debian.org/1022080):

[qcengine] appears to use `yaml.load()` without specifying a `Loader=`
argument, which will become an error in pyyaml version 6. This should
have emitted a warning message since version 5.1 (from 2019).

In most cases this can be fixed by replacing `yaml.load` with
`yaml.safe_load`, unless the ability for yaml to create arbitrary python
objects is desirable.

Found in qcengine/config.py _load_defaults():
https://github.com/MolSSI/QCEngine/blob/master/qcengine/config.py#L193

To Reproduce
I haven't tried to reproduce it, but I assume trying to use yaml will yield the warning.

Expected behavior
No deprecation warnings