[BEAM-6896] Loosen PyYAML dependency.

[robertwb]

Also update this to be a test-only dependency, as it's only used for tests.

There were no relevant API changes with the last major release.

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Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---		---	---
Java
Python		---			---	---
XLang	---	---	---		---	---	---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website
Non-portable
Portable	---		---	---

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[robertwb]

R: @yifanzou