Mark test_installation xfail on Cygwin CI

Together with gitpython-developers#2007, this works around gitpython-developers#2004, allowing all tests to
pass on Cygwin CI.

In gitpython-developers#2007, installation of the environment in which tests run was
fixed by downloading and running the `get-pip.py` bootstrap script.
If we were to modify our helper that sets up the (separate) virtual
environment in `test_installation` so that it does the same thing
(or conditionally does so on CI, since the problem does not seem to
happen in local installations), that would likely "fix" this more
thoroughly, allowing the test to pass.

But part of the goal of the installation test is to test that
installation works in a typical environment on the platform it runs
on. So it is not obivous that making it pass in that way would be
an improvement compared to marking it `xfail` with the exception
type that occurs due to gitpython-developers#2004. So this just does that, for now.

Author: EliahKagan

Diff for: test/test_installation.py

@@ -4,11 +4,19 @@
 import ast
 import os
 import subprocess
+import sys
+
+import pytest
 
 from test.lib import TestBase, VirtualEnvironment, with_rw_directory
 
 
 class TestInstallation(TestBase):
+    @pytest.mark.xfail(
+        sys.platform == "cygwin" and "CI" in os.environ,
+        reason="Trouble with pip on Cygwin CI, see issue #2004",
+        raises=subprocess.CalledProcessError,
+    )
     @with_rw_directory
     def test_installation(self, rw_dir):
         venv = self._set_up_venv(rw_dir)