Special-case Python 3.7 for OSes we can install it on

This is analogous to the 3.7-related CI change in gitdb that was
part of gitpython-developers/gitdb#114, as
to part of gitpython-developers/smmap#58.

Since some tests are not yet passing on 3.13, this does not add
3.13 to CI, nor to the documentation of supported versions in
`setup.py`. Note that the list there is not enforced; GitPython can
already be installed on Python 3.13 and probably *mostly* works.

(See gitpython-developers#1955
for details on other changes that should be made to fully support
running GitPython on Python 3.13.)

.github/workflows/pythonpackage.yml

@@ -11,15 +11,19 @@ permissions:
 jobs:
   build:
     strategy:
-      fail-fast: false
       matrix:
-        os: ["ubuntu-22.04", "macos-latest", "windows-latest"]
-        python-version: ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12"]
-        exclude:
-        - os: "macos-latest"
-          python-version: "3.7"
+        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
+          python-version: "3.7"
+          experimental: false
+        - os: windows-latest
+          python-version: "3.7"
+          experimental: false
+
+      fail-fast: false
 
     runs-on: ${{ matrix.os }}