fix sphinx build

Author: jlwalke2

tox.ini

@@ -59,6 +59,8 @@ deps =
 #    tests: lightgbm ; platform_system != "Darwin"  # lightgmb seems to have build issues on MacOS
     codecov: codecov >= 1.4.0
     doc: sphinx == 1.8
+# doc skips install, so explicitly add minimum packages
+    doc: pyyaml
 
 setenv =
     tests: SASCTL_USER_NAME=dummy