ci: Add an env variable with the default Python version

When not using a matrix, we want to use the same Python version for all
jobs and steps for consistency. To make it easier to update, we add an
`env` variable with the default version so we can use it where it is
needed.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

.github/workflows/ci.yaml

@@ -2,6 +2,9 @@ name: frequenz-channels-python
 
 on: [pull_request, push, workflow_dispatch]
 
+env:
+  DEFAULT_PYTHON_VERSION: "3.10"
+
 jobs:
   test:
     strategy:
@@ -49,7 +52,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v4
         with:
-          python-version: "3.10"
+          python-version: ${{ env.DEFAULT_PYTHON_VERSION }}
 
       - name: Install build dependencies
         run: |
@@ -79,7 +82,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v4
         with:
-          python-version: "3.10"
+          python-version: ${{ env.DEFAULT_PYTHON_VERSION }}
 
       - name: Install build dependencies
         run: |
@@ -156,7 +159,7 @@ jobs:
         if: steps.mike-metadata.outputs.version
         uses: actions/setup-python@v4
         with:
-          python-version: "3.10"
+          python-version: ${{ env.DEFAULT_PYTHON_VERSION }}
 
       - name: Install build dependencies
         if: steps.mike-metadata.outputs.version