CI: Don't invoke linters on Python 3.9

pyproject-fmt 2.8.0 fails on Python 3.9.

  thread '<unnamed>' panicked at common/src/create.rs:22:5:

Author: amotl

.github/workflows/tests.yml

@@ -71,6 +71,7 @@ jobs:
         npm install
 
     - name: Check code style
+      if: ${{ matrix.python-version != '3.9' }}
       run: |
         poe lint