Add support for system_site_packages matching tox own behavior

Fixes: #109

# Conflicts:
#	tests/test_tox_uv_venv.py

src/tox_uv/_venv.py

@@ -10,6 +10,7 @@
 from platform import python_implementation
 from typing import TYPE_CHECKING, Any, Literal, Optional, Type, cast  # noqa: UP035
 
+from tox.config.loader.str_convert import StrConvert
 from tox.execute.local_sub_process import LocalSubProcessExecutor
 from tox.execute.request import StdinSource
 from tox.tox_env.errors import Skip
@@ -56,6 +57,15 @@ def register_config(self) -> None:
             default=False,
             desc="add seed packages to the created venv",
         )
+        self.conf.add_config(
+            keys=["system_site_packages", "sitepackages"],
+            of_type=bool,
+            default=lambda conf, name: StrConvert().to_bool(  # noqa: ARG005
+                self.environment_variables.get("VIRTUALENV_SYSTEM_SITE_PACKAGES", "False"),
+            ),
+            desc="create virtual environments that also have access to globally installed packages.",
+        )
+
         # The cast(...) might seems superfluous but removing it makes mypy crash. The problem isy on tox typing side.
         self.conf.add_config(
             keys=["uv_python_preference"],
@@ -195,6 +205,8 @@ def create_python_env(self) -> None:
             cmd.append("-v")
         if self.conf["uv_seed"]:
             cmd.append("--seed")
+        if self.conf["system_site_packages"]:
+            cmd.append("--system-site-packages")
         if self.conf["uv_python_preference"]:
             cmd.extend(["--python-preference", self.conf["uv_python_preference"]])
         cmd.append(str(self.venv_dir))

tests/test_tox_uv_venv.py

@@ -133,6 +133,12 @@ def test_uv_venv_spec_full_implementation(
     assert f"no interpreter found for {expected_name} 9.99" in result.err.lower()
 
 
+def test_uv_venv_system_site_packages(tox_project: ToxProjectCreator) -> None:
+    project = tox_project({"tox.ini": "[testenv]\npackage=skip\nsystem_site_packages=true"})
+    result = project.run("-vv")
+    result.assert_success()
+
+
 @pytest.fixture
 def other_interpreter_exe() -> pathlib.Path:  # pragma: no cover
     """Returns an interpreter executable path that is not the exact same as `sys.executable`.