Use ruamel.yaml

[victorlin]

Description of proposed changes

The main motivation is to have errors on duplicate keys, which PyYAML silently allows due to a longstanding bug.

The other notable difference is a switch from YAML 1.1 to YAML 1.2 (latest).

Related issue(s)

Follow-up to #2040 (comment)

Checklist

• Automated checks pass
• Check if you need to add a changelog message
• Check if you need to add tests
• Check if you need to update docs

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 73.27%. Comparing base (7c7ac40) to head (a9e8d2f).

Files with missing lines	Patch %	Lines
augur/validate.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2046   +/-   ##
=======================================
  Coverage   73.26%   73.27%           
=======================================
  Files          86       86           
  Lines       10765    10767    +2     
  Branches     2099     2099           
=======================================
+ Hits         7887     7889    +2     
  Misses       2503     2503           
  Partials      375      375           

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