Fix yaml warnings by using safe_load

[erikjohnston]

If you don't then it complains loudly to stderr. We can just use the
full loader as all of our yaml loading happens from trusted source (e.g.
config)

[codecov]

Codecov Report

Merging #4869 into develop will decrease coverage by 0.31%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           develop   #4869      +/-   ##
==========================================
- Coverage    78.01%   77.7%   -0.32%     
==========================================
  Files          326     326              
  Lines        34058   34341     +283     
  Branches      5621    5660      +39     
==========================================
+ Hits         26572   26683     +111     
- Misses        5867    6025     +158     
- Partials      1619    1633      +14

[hawkowl]

@erikjohnston is there any reason why we can't just use load_safe?

[erikjohnston]

Probably not, I was just hoping to avoid having to think about it. Though given this doesn't work with old pyyaml that's probably the way to go

[erikjohnston]

I've updated this to now use safe_load