The previous version (`3.12`) is 7 years old and does not support the
newest Python 3 versions. This causes issues to move certain test
targets (which depends on `pyyaml`) to Python 3 when some CI environment
(e.g. `arm64v8/debian:11`) does not have Python 2 installed. And in
general, we should move away from Python 2. Thus, updated `pyyaml` to
the latest version.
This hopefully should also fix the
`prod:grpc/core/master/linux/arm64/grpc_bazel_test_c_cpp` job breakage.
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