Skip python 3.6 for pre-commit

Python 3.6 will fail with pre-commit setup:

  Complete output (6 lines):
  Traceback (most recent call last):
    File "<string>", line 1, in <module>
    File "/home/runner/.cache/pre-commit/repozbn0y53a/setup.py", line 1
      from __future__ import annotations
      ^
  SyntaxError: future feature annotations is not defined

The pre-commit project has dropped support on 3.6:

pre-commit/pre-commit#2215

Signed-off-by: Wayne Sun <gsun@redhat.com>

.github/workflows/pull_request.yml

@@ -29,3 +29,4 @@ jobs:
 
       - name: Pre Commit Checks
         uses: pre-commit/action@v3.0.0
+        if: ${{ matrix.python-version != '3.6' }}