Remove stuck CI checks with Python 3.6

The checks with Python 3.6 seem to get stuck at a very early stage  with no
output at all. Canceling/restarting didn't help when I tried it on
2025-08-12 and 2025-08-13.

Considering Python 3.6 is EOL anyway it makes not much sense to invest more
effort here. So I'm simply removing these checks.

Author: Martchus

.github/workflows/tox.yml

@@ -11,7 +11,6 @@ jobs:
       fail-fast: false
       matrix:
         python-version:
-          - "3.6"
           - "3.8"
           - "3.9"
           - "3.10"
@@ -21,7 +20,6 @@ jobs:
         include:
           - os: "ubuntu-latest"
           - os: "ubuntu-20.04"
-            python-version: "3.6"
 
     steps:
       - uses: actions/checkout@v4