python: Utilis setup-python's new python-version-file

This way we only need to edit `.python-version` and all the GitHub actions
will automatically use that version.

.github/workflows/pr-checks.yml

@@ -52,10 +52,10 @@ jobs:
     ####################################################################
     # Get Python set up
     ####################################################################
-    - name: Set up Python 3.10
+    - name: Set up Python
       uses: actions/setup-python@v4
       with:
-        python-version: "3.10"
+        python-version-file: '.python-version'
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip

.github/workflows/push-checks.yml

@@ -20,10 +20,10 @@ jobs:
     - uses: actions/checkout@v3
       with:
         fetch-depth: 0
-    - name: Set up Python 3.10
+    - name: Set up Python
       uses: actions/setup-python@v4
       with:
-        python-version: "3.10"
+        python-version-file: '.python-version'
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip

.github/workflows/windows-build.yml

@@ -22,7 +22,7 @@ jobs:
 
       - uses: actions/setup-python@v4
         with:
-          python-version: "3.10.4"
+          python-version-file: '.python-version'
           architecture: "x86"
 
       - name: Install python tools

docs/Releasing.md

@@ -433,13 +433,13 @@ If you are making a pre-release then:
 
 When changing the Python version (Major.Minor.Patch) used:
 
-1. Change the contents of `.python-version` so that pyenv notices.
+1. Change the contents of `.python-version` so that pyenv notices.  All of
+  the GitHub workflows now reference this via the `setup-python`
+  `python-version-file` directive.
 
 1. Any version change:
 
-   1. `.github/workflows/windows-build.yml` needs updating to have the GitHub
-   based build use the correct version.
-   2. `ChangeLog.md` - The `We now test against, and package with, Python
+   1. `ChangeLog.md` - The `We now test against, and package with, Python
        M.m.P.` line.
 
 1. Major or Minor level changes:
@@ -449,5 +449,3 @@ When changing the Python version (Major.Minor.Patch) used:
        pythonXX.dll file.
     3. `.pre-commit-config.yaml` will need the `default_language_version`
        section updated to the appropriate version.
-    4. All `.github/workflows/` files will need to be citing the correct
-       version in any `uses: actions/setup-python` section.