Remove {pytester,testdir}.Session aliases

This causes `Session` documentation to be rendered again in full under
`pytester` and `testdir` in the API Reference. I tried but couldn't get
sphinx to hide it.

Since it's a pretty odd thing to have (should just use
`pytest.Session`), and I couldn't find any plugin which uses this, let's
just remove it.

src/_pytest/pytester.py

@@ -950,8 +950,6 @@ class Pytester:
                 f'example "{example_path}" is not found as a file or directory'
             )
 
-    Session = Session
-
     def getnode(
         self, config: Config, arg: Union[str, "os.PathLike[str]"]
     ) -> Optional[Union[Collector, Item]]:
@@ -1529,7 +1527,6 @@ class Testdir:
 
     CLOSE_STDIN: "Final" = Pytester.CLOSE_STDIN
     TimeoutExpired: "Final" = Pytester.TimeoutExpired
-    Session: "Final" = Pytester.Session
 
     def __init__(self, pytester: Pytester, *, _ispytest: bool = False) -> None:
         check_ispytest(_ispytest)

testing/python/collect.py

@@ -7,6 +7,7 @@ from typing import Dict
 import _pytest._code
 import pytest
 from _pytest.config import ExitCode
+from _pytest.main import Session
 from _pytest.monkeypatch import MonkeyPatch
 from _pytest.nodes import Collector
 from _pytest.pytester import Pytester
@@ -294,7 +295,7 @@ class TestFunction:
         from _pytest.fixtures import FixtureManager
 
         config = pytester.parseconfigure()
-        session = pytester.Session.from_config(config)
+        session = Session.from_config(config)
         session._fixturemanager = FixtureManager(session)
 
         return pytest.Function.from_parent(parent=session, **kwargs)