Python: Bump minor version in GH workflows & update Releasing.md

I used Releasing.md as reference for what would need the version
updating, but it didn't say anything about all the workflows, it now
does.

.github/workflows/pr-checks.yml

@@ -52,10 +52,10 @@ jobs:
     ####################################################################
     # Get Python set up
     ####################################################################
-    - name: Set up Python 3.9
+    - name: Set up Python 3.10
       uses: actions/setup-python@v2.3.1
       with:
-        python-version: 3.9
+        python-version: 3.10
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip

.github/workflows/push-checks.yml

@@ -20,10 +20,10 @@ jobs:
     - uses: actions/checkout@v2.4.0
       with:
         fetch-depth: 0
-    - name: Set up Python 3.9
+    - name: Set up Python 3.10
       uses: actions/setup-python@v2.3.1
       with:
-        python-version: 3.9
+        python-version: 3.10
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip