Drop py36 testing from the CI

The testing runners use Python 3.8 (Focal), hence no need to keep py36
alive.

.github/workflows/tox.yaml

@@ -5,31 +5,6 @@ on:
   - pull_request
 
 jobs:
-  build_old_versions:
-    runs-on: ubuntu-20.04
-    strategy:
-      matrix:
-        python-version: ['3.6']
-
-    steps:
-    - uses: actions/checkout@v1
-    - name: Set up Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v2
-      with:
-        python-version: ${{ matrix.python-version }}
-    - name: Install dependencies
-      run: |
-        python -m pip install --upgrade pip
-        pip install codecov tox tox-gh-actions
-    - name: Lint with tox
-      run: tox -e pep8
-    - name: Test with tox
-      run: tox -e py
-    - name: Codecov
-      run: |
-        set -euxo pipefail
-        codecov --verbose --gcov-glob unit_tests/*
-
   build:
     runs-on: ubuntu-latest
     strategy: