CI: Disable code style checks on Python 3.6, it is not supported by ruff

Author: amotl

.github/workflows/test.yml

@@ -40,6 +40,7 @@ jobs:
           python -m pip install --editable=.[test,develop]
 
       - name: Check code style
+        if: matrix.python-version != '3.6' && matrix.python-version != 'pypy-3.6'
         run: |
           poe check-style
 

setup.cfg

@@ -88,7 +88,7 @@ develop =
     build<1
     poethepoet<1
     pip-review<2  # Use `pip-review --local --interactive` to upgrade outdated packages.
-    ruff
+    ruff;python_version>='3.7'
 
 
 [options.packages.find]