Ensure values are str types when converting from YAML to Markdown

[mcdonnnj]

🗣 Description

This pull request updates portions of the yml2md command so that any values where we check the len() is checked for truthiness and converted with str() first. It also converts a piece of error handling from manually printing to stderr to instead use the logging library appropriately.

💭 Motivation and context

When a person who is unfamiliar with YAML is updating an entry they may see no problem with the following:

  affected_versions:
    - < 1.4
    - 1.5
    - '> 1.6'

However, the second value will be read as a float value instead of as a str value. This will cause the yml2md command to fail with a TypeError when processing this entry.

🧪 Testing

Automated tests pass. When run with the sample version data above the v1.1.1 branch fails with a TypeError as expected. When used with this branch the conversion completes with no issues.

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All new and existing tests pass.

[dav3r]

This makes sense to me. String the things. 🚌

[coveralls]

Pull Request Test Coverage Report for Build 1741031944

• 0 of 2 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 29.459%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ymlmd/yml2md.py	0	2	0.0%

Totals
Change from base Build 1740961890:	0.0%
Covered Lines:	103
Relevant Lines:	344

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💛 - Coveralls