Ensure running py3.6 tests only where it is available

Python 3.6 reached its end of life on Dec 12, 2021 and GitHub
actions stopped supporting it on its latest Ubuntu systems (>20.04).
From now, Python3.6 will be tested only on Ubuntu 20.04 (beside
windows-latest and macos-latest).

.github/workflows/main.yml

@@ -7,6 +7,12 @@ jobs:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
         python-version: [3.6, 3.7, 3.8, 3.9, '3.10', '3.11', 'pypy-3.7']
+        exclude:
+          - python-version: "3.6"
+            os: ubuntu-latest
+        include:
+          - python-version: "3.6"
+            os: ubuntu-20.04
     runs-on: ${{ matrix.os }}
     steps:
       - run: git config --global core.autocrlf input