message: Add PyYAML to address 2.4 build issue
diff: @@ -147,6 +147,8 @@ function install_ubuntu_16_pip_deps {
  "${PIP_CMD}" install --user 'portpicker ~= 1.3.1'
  "${PIP_CMD}" install --user 'scipy ~= 1.5.2'
  # LINT.ThenChange(:mac_pip_installations)
  + # Need to be addressed later. Unblocking 2.4 branchcut
  + "${PIP_CMD}" install --user 'PyYAML ~= 5.3.1'
  }
  function install_macos_pip_deps {
  