Test changed setup, and Python 3.12, on CI

Key changes:

- Update the two CI workflows to install the project and its
  dependencies in accordance with the changed recommendations in
  README.md. (This is to test that those recommendations work,
  which the changed test_installation test case partially but not
  completely tests. The old approach to installation still works
  too, so this change on CI is not required to keep CI working.)

- Add Python 3.12 to the CI test matrix in pythonpackage.yml,
  testing it on Ubuntu. (The Cygwin workflow still tests only 3.9.)

Maintenance changes, made to avoid decreasing readability with the
other changes (and hopefully even increase it somewhat):

- Separate commands into more steps, grouping them by more specific
  purposes.

- Decrease the ways the two workflows differ from each other that
  do not represent actual intended behavioral differences. This is
  to make the important differences easier to stop, and to make it
  easier to determine when the same change has or has not been made
  to both workflows.

.github/workflows/cygwin-test.yml

@@ -12,38 +12,61 @@ jobs:
       SHELLOPTS: igncr
       TMP: "/tmp"
       TEMP: "/tmp"
+    defaults:
+      run:
+        shell: bash.exe -eo pipefail -o igncr "{0}"
 
     steps:
     - name: Force LF line endings
       run: git config --global core.autocrlf input
+
     - uses: actions/checkout@v4
       with:
         fetch-depth: 9999
+
     - uses: cygwin/cygwin-install-action@v4
       with:
         packages: python39 python39-pip python39-virtualenv git
+
+    - name: Show python and git versions
+      run: |
+        set -x
+        /usr/bin/python --version
+        /usr/bin/git version
+
     - name: Tell git to trust this repo
-      shell: bash.exe -eo pipefail -o igncr "{0}"
       run: |
           /usr/bin/git config --global --add safe.directory "$(pwd)"
-    - name: Install dependencies and prepare tests
-      shell: bash.exe -eo pipefail -o igncr "{0}"
+
+    - name: Prepare this repo for tests
       run: |
         set -x
-        /usr/bin/python -m pip install --upgrade pip setuptools wheel
-        /usr/bin/python --version; /usr/bin/git --version
+
         /usr/bin/git submodule update --init --recursive
         /usr/bin/git fetch --tags
-        /usr/bin/python -m pip install -r requirements.txt
-        /usr/bin/python -m pip install -r test-requirements.txt
         TRAVIS=yes ./init-tests-after-clone.sh
+
+    - name: Further prepare git configuration for tests
+      run: |
+        set -x
+
         /usr/bin/git config --global user.email "travis@ci.com"
         /usr/bin/git config --global user.name "Travis Runner"
         # If we rewrite the user's config by accident, we will mess it up
         # and cause subsequent tests to fail
         cat test/fixtures/.gitconfig >> ~/.gitconfig
+
+    - name: Update PyPA packages
+      run: |
+        set -x
+        /usr/bin/python -m pip install --upgrade pip setuptools wheel
+
+    - name: Install project and test dependencies
+      run: |
+        set -x
+        /usr/bin/python -m pip install ".[test]"
+
     - name: Test with pytest
-      shell: bash.exe -eo pipefail -o igncr "{0}"
       run: |
+        set -x
         /usr/bin/python -m pytest
-      continue-on-error: false

.github/workflows/pythonpackage.yml

@@ -15,42 +15,70 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [3.7, 3.8, 3.9, "3.10", "3.11"]
+        python-version: ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12"]
+        include:
+          - experimental: false
+          - python-version: "3.12"
+            experimental: true
 
     steps:
     - uses: actions/checkout@v4
       with:
         fetch-depth: 9999
+
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v4
       with:
         python-version: ${{ matrix.python-version }}
-    - name: Install dependencies and prepare tests
+        allow-prereleases: ${{ matrix.experimental }}
+
+    - name: Show python and git versions
+      run: |
+        set -x
+        python --version
+        git version
+
+    - name: Prepare this repo for tests
       run: |
         set -x
 
-        python -m pip install --upgrade pip setuptools wheel
-        python --version; git --version
         git submodule update --init --recursive
         git fetch --tags --force
-
-        pip install -r requirements.txt
-        pip install -r test-requirements.txt
         TRAVIS=yes ./init-tests-after-clone.sh
 
+    - name: Prepare git configuration for tests
+      run: |
+        set -x
+
         git config --global user.email "travis@ci.com"
         git config --global user.name "Travis Runner"
         # If we rewrite the user's config by accident, we will mess it up
         # and cause subsequent tests to fail
         cat test/fixtures/.gitconfig >> ~/.gitconfig
 
+    - name: Update PyPA packages
+      run: |
+        set -x
+
+        python -m pip install --upgrade pip
+        if pip freeze --all | grep --quiet '^setuptools=='; then
+            # Python prior to 3.12 ships setuptools. Upgrade it if present.
+            python -m pip install --upgrade setuptools
+        fi
+        python -m pip install --upgrade wheel
+
+    - name: Install project and test dependencies
+      run: |
+        set -x
+        pip install ".[test]"
+
     - name: Check types with mypy
-      # With new versions of pypi new issues might arise. This is a problem if there is nobody able to fix them,
-      # so we have to ignore errors until that changes.
-      continue-on-error: true
       run: |
         set -x
         mypy -p git
+      # With new versions of mypy new issues might arise. This is a problem if there is nobody able to fix them,
+      # so we have to ignore errors until that changes.
+      continue-on-error: true
 
     - name: Test with pytest
       run: |