Refactor/yaml ruamel

[hulkoba]

Checklist

• Added a news entry

Unify usage of ruamel and yaml

This PR refactors the codebase to use ruamel instead of yaml.

There is one file left, where a refactor does not make sense in my opinion: conda-smithy/variant_algebra.py

The first problem is, that ruamel returns None for empty values, so we had to override the variants.trim_empty_keys(content) to make it work.
Secondly, yaml returns numbers as string and ruamel does not. Parsing the version numbers to float causes several issues because the trailing zeros are not correct.
This is a rabbit hole 🐇 and we decided to keep it as it is, finally.

[coveralls]

Pull Request Test Coverage Report for Build 9613167164

Details

• 22 of 22 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 67.914%

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Totals
Change from base Build 9612517554:	0.04%
Covered Lines:	2799
Relevant Lines:	4042

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💛 - Coveralls