Stop testing Python 3.6 to please CI

The actual removal will be done in another commit.

Author: pquentin

.ci/test-matrix.yml

@@ -5,7 +5,6 @@ TEST_SUITE:
   - platinum
 
 PYTHON_VERSION:
-  - "3.6"
   - "3.7"
   - "3.8"
   - "3.9"

.github/workflows/ci.yml

@@ -43,10 +43,6 @@ jobs:
         nox-session: [""]
         runs-on: ["ubuntu-latest"]
         include:
-          - python-version: 3.6
-            experimental: false
-            nox-session: test-3.6
-            runs-on: "ubuntu-20.04"
           - python-version: 3.11-dev
             experimental: true
             nox-session: test-3.11