update sphinx build env

Author: jlwalke2

.github/workflows/build-test-deploy.yml

@@ -98,7 +98,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v2
         with:
-          python-version: 3.6
+          python-version: 3.9
 
       - name: Setup environment
         run: |

tox.ini

@@ -30,7 +30,6 @@ python =
 [testenv]
 skip_install =
     clean: true
-    doc: true
 
 basepython =
     py36: python3.6
@@ -54,7 +53,7 @@ deps =
     tests: xgboost == 0.82
 #    tests: lightgbm ; platform_system != "Darwin"  # lightgmb seems to have build issues on MacOS
 # doc skips install, so explicitly add minimum packages
-    doc: sphinx == 1.8
+    doc: sphinx
     doc: pyyaml
 
 setenv =