Tune CI matrix adjustments so reports are clearer

Since gitpython-developers#1987, test jobs from `pythonpackage.yml` appear in an
unintuitive order, and some show an extra bool matrix variable in
their names while others don't (this corresponds to `experimental`,
which was always set to some value, but was set in different ways).

This fixes that by:

- Listing all tested versions, rather than introducing some in an
  `include` key. (The `include:`-introduced jobs didn't distinguish
  between originally-present matrix variables and those that are
  introduced based on the values of the original ones.)

- Replacing `os` with `os-type`, which has only the first part of
  the value for `runs-on:` (e.g., `ubuntu`), and adding `os-ver`
  to each matrix job, defaulting it to `latest`, but using `22.04`
  for Python 3.7 on Ubuntu.

This should also naturally extend to adding 3.13, with or without
setting `continue-on-error` to temporarily work around the problems
obseved in gitpython-developers#1955, but nothing 3.13-related is done in this commit.

.github/workflows/pythonpackage.yml

@@ -12,20 +12,21 @@ jobs:
   build:
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: ["3.8", "3.9", "3.10", "3.11", "3.12"]
-        include:
-        - experimental: false
-        - os: ubuntu-22.04
+        os-type: [ubuntu, macos, windows]
+        python-version: ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12"]
+        exclude:
+        - os-type: macos
           python-version: "3.7"
-          experimental: false
-        - os: windows-latest
+        include:
+        - os-ver: latest
+        - os-type: ubuntu
           python-version: "3.7"
-          experimental: false
+          os-ver: "22.04"
+        - experimental: false
 
       fail-fast: false
 
-    runs-on: ${{ matrix.os }}
+    runs-on: ${{ matrix.os-type }}-${{ matrix.os-ver }}
 
     defaults:
       run:
@@ -43,7 +44,7 @@ jobs:
         allow-prereleases: ${{ matrix.experimental }}
 
     - name: Set up WSL (Windows)
-      if: startsWith(matrix.os, 'windows')
+      if: matrix.os-type == 'windows'
       uses: Vampire/[email protected]
       with:
         distribution: Alpine
@@ -80,7 +81,7 @@ jobs:
 
     # For debugging hook tests on native Windows systems that may have WSL.
     - name: Show bash.exe candidates (Windows)
-      if: startsWith(matrix.os, 'windows')
+      if: matrix.os-type == 'windows'
       run: |
         set +e
         bash.exe -c 'printenv WSL_DISTRO_NAME; uname -a'