Drop python 3.6 support (#10468)

.github/workflows/builddoc.yml

@@ -11,7 +11,7 @@ jobs:
     - name: Set up Python
       uses: actions/setup-python@v1
       with:
-        python-version: 3.6
+        python-version: 3.8
     - name: Install dependencies
       run:  |
         sudo apt update

.github/workflows/lint.yml

@@ -15,7 +15,7 @@ jobs:
     - name: Set up Python
       uses: actions/setup-python@v1
       with:
-        python-version: 3.6
+        python-version: 3.8
     - name: Install dependencies
       run:  pip install -U tox
     - name: Run Tox

.github/workflows/main.yml

@@ -10,8 +10,6 @@ jobs:
       fail-fast: false
       matrix:
         include:
-        - python: "3.6"
-          docutils: du14
         - python: "3.7"
           docutils: du15
         - python: "3.8"