Install WSL system on CI for hook tests

This makes three of the four hook-related tests pass instead of
failing, and changes the way the fourth fails.

GitPython uses bash.exe to run hooks that are (or appear to be)
shell scripts. On many Windows systems, this is the bash.exe in the
system32 directory, which delegates to bash in a WSL system if at
least one such system is installed (for the current user), and
gives an error otherwise. It may be a bug that GitPython ends up
using this bash.exe when WSL is installed but no WSL systems exist,
since that is actually a fairly common situation. One place that
happened was on the GitHub Actions runners used for Windows jobs.
Those runners have WSL available, and are capable of running WSL 1
systems (not currently WSL 2 systems), but no WSL systems were
actually installed.

This commit fixes that cause of failure, for all four tests it
happened in, by setting up a Debian WSL system on the test runner.
(This increases the amount of time it takes Windows jobs to run,
but that might be possible to improve on.) Three of those four
tests now pass, while the other fails for another reason.

The newly passing tests are:

- test/test_index.py::TestIndex::test_commit_msg_hook_fail
- test/test_index.py::TestIndex::test_pre_commit_hook_fail
- test/test_index.py::TestIndex::test_pre_commit_hook_success

The test that still fails, but differently, is:

- test/test_index.py::TestIndex::test_commit_msg_hook_success

I had previously found that test to fail on a local WSL 2 system,
and attempted to add a suitable xfail marking in 881456b (gitpython-developers#1679).
But the condition I wrote there *appears* to have a bug related to
the different orders in which subproces.Popen and shutil.which find
executables, causing it not always to detect when the WSL-related
bash.exe is the one the Popen call in git.index.fun.run_commit_hook
will use.

.github/workflows/pythonpackage.yml

@@ -35,6 +35,12 @@ jobs:
         python-version: ${{ matrix.python-version }}
         allow-prereleases: ${{ matrix.experimental }}
 
+    - name: Set up WSL
+      if: startsWith(matrix.os, 'windows')
+      uses: Vampire/setup-wsl@v2.0.2
+      with:
+        distribution: Debian
+
     - name: Prepare this repo for tests
       run: |
         ./init-tests-after-clone.sh