Fix yaml load to be compatible with pyyaml 5.1

[mraspaud]

Pyyaml 5.1 deprecated the use of unsafe loading as default, making satpy config loading shaky. This PR restores the expected behaviour in satpy

Here are some explanations about the pyyaml change:
https://github.com/yaml/pyyaml/wiki/PyYAML-yaml.load(input)-Deprecation

• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff

[codecov]

Codecov Report

Merging #655 into master will decrease coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #655      +/-   ##
==========================================
- Coverage    78.4%   78.39%   -0.02%     
==========================================
  Files         138      138              
  Lines       19990    20013      +23     
==========================================
+ Hits        15673    15689      +16     
- Misses       4317     4324       +7

Impacted Files	Coverage Δ
satpy/config.py	47.91% <42.85%> (+0.09%)	⬆️
satpy/plugin_base.py	88% <60%> (-7.24%)	⬇️
satpy/composites/__init__.py	65.55% <66.66%> (-0.09%)	⬇️
satpy/readers/__init__.py	94.54% <66.66%> (+0.66%)	⬆️
satpy/readers/yaml_reader.py	92.26% <71.42%> (-0.35%)	⬇️
satpy/writers/__init__.py	85.82% <81.81%> (-0.37%)	⬇️
satpy/scene.py	88.95% <0%> (+0.18%)	⬆️

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[coveralls]

Coverage decreased (-0.01%) to 78.394% when pulling 0fc351c on mraspaud:fix-yaml-load into ee02de0 on pytroll:master.

[coveralls]

Coverage decreased (-0.01%) to 78.394% when pulling 0fc351c on mraspaud:fix-yaml-load into ee02de0 on pytroll:master.