chore(ci): add .python-version file for setup-python@v4

the new actions/setup-python@v4 requires either a python-version or python-version-file param
(there is no default version)

use python-version-file so we can point to a common file in multiple workflows

.github/workflows/.python-version

@@ -0,0 +1 @@
+3.9

.github/workflows/pre-commit.yml

@@ -9,5 +9,9 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
+
       - uses: actions/setup-python@v4
+        with:
+          python-version-file: .github/workflows/.python-version
+
       - uses: pre-commit/action@v3.0.0

.github/workflows/tests-pytest.yml

@@ -16,7 +16,7 @@ jobs:
 
       - uses: actions/setup-python@v4
         with:
-          python-version: '3.9'
+          python-version-file: .github/workflows/.python-version
           cache: pip
           cache-dependency-path: '**/requirements.txt'