CI: back off the agressive timeout

Precise number does not matter that much, so lets not stop potentially working tests.
The point was to cut off well before 6 hours, so any small number will do.

.github/workflows/tox.yml

@@ -9,7 +9,7 @@ jobs:
   tox:
     name: ${{ matrix.os }} / ${{ matrix.python-version }}
     # https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idtimeout-minutes
-    timeout-minutes: 10
+    timeout-minutes: 20
     runs-on: ${{ matrix.os }}
     strategy:
       fail-fast: false