Revert "Cleanup: delete yaml_load_safely (#11285)"

[ericholscher]

This reverts commit 791643f.

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: readthedocs/doc_builder/backends/mkdocs.py

Function	Unhandled Issue
build	AttributeError: 'NoneType' object has no attribute 'get' readthedocs.projects.tasks.builds.update_d... Event Count: 2

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