Move Session.startdir to legacypath plugin

src/_pytest/legacypath.py

@@ -362,6 +362,16 @@ def Config_inifile(self: pytest.Config) -> Optional[LEGACY_PATH]:
     return legacy_path(str(self.inipath)) if self.inipath else None
 
 
+def Session_stardir(self: pytest.Session) -> LEGACY_PATH:
+    """The path from which pytest was invoked.
+
+    Prefer to use ``startpath`` which is a :class:`pathlib.Path`.
+
+    :type: LEGACY_PATH
+    """
+    return legacy_path(self.startpath)
+
+
 def pytest_configure(config: pytest.Config) -> None:
     mp = pytest.MonkeyPatch()
     config.add_cleanup(mp.undo)
@@ -399,3 +409,6 @@ def pytest_configure(config: pytest.Config) -> None:
     )
     mp.setattr(pytest.Config, "rootdir", property(Config_rootdir), raising=False)
     mp.setattr(pytest.Config, "inifile", property(Config_inifile), raising=False)
+
+    # Add Session.startdir property.
+    mp.setattr(pytest.Session, "startdir", property(Session_stardir), raising=False)

src/_pytest/main.py

@@ -25,8 +25,6 @@ import attr
 import _pytest._code
 from _pytest import nodes
 from _pytest.compat import final
-from _pytest.compat import LEGACY_PATH
-from _pytest.compat import legacy_path
 from _pytest.config import Config
 from _pytest.config import directory_arg
 from _pytest.config import ExitCode
@@ -504,16 +502,6 @@ class Session(nodes.FSCollector):
         """
         return self.config.invocation_params.dir
 
-    @property
-    def stardir(self) -> LEGACY_PATH:
-        """The path from which pytest was invoked.
-
-        Prefer to use ``startpath`` which is a :class:`pathlib.Path`.
-
-        :type: LEGACY_PATH
-        """
-        return legacy_path(self.startpath)
-
     def _node_location_to_relpath(self, node_path: Path) -> str:
         # bestrelpath is a quite slow function.
         return self._bestrelpathcache[node_path]