CI: Don't run linter on Python 3.7

Recent Black 24 is not available for Python 3.7, and Black 23 formats
code differently.

Author: amotl

.github/workflows/tests.yml

@@ -54,7 +54,7 @@ jobs:
         pip install --editable=.[test,develop]
 
     - name: Check code style
-      if: matrix.python-version != '3.6'
+      if: matrix.python-version != '3.6' && matrix.python-version != '3.7'
       run: |
         poe lint