Install PyYaml for test-matrix.py

We have an implicit dependency on PyYaml in test-matrix.py, install it
prior to running test-matrix.py.

Co-authored-by: Aaron Rosen <aaronorosen@gmail.com>
Change-Id: I7d16a73d5d73e0901a54309ef2e7c4b68a5abd3f
Merge "Install PyYaml for test-matrix.py"