doc: exclude Testdir.Session (#519)

* doc: exclude Testdir.Session

`Testdir.Session` appears to be there for convenience only, and is used
in internal tests.
This patch excludes it from `Testdir`'s documentation, which causes
duplicate cross-references for `Session` in Sphinx.

* Move setting of Testdir.Session up

Author: blueyed

doc/en/reference.rst

@@ -512,8 +512,9 @@ To use it, include in your top-most ``conftest.py`` file:
 
 
 
-.. autoclass:: Testdir()
+.. autoclass:: Testdir
     :members:
+    :exclude-members: Session
 
 .. autoclass:: RunResult()
     :members:

src/_pytest/pytester/__init__.py

@@ -601,6 +601,8 @@ class Testdir(Generic[AnyStr]):
     CLOSE_STDIN = CLOSE_STDIN
     """Sentinel to close stdin."""
 
+    Session = Session
+
     class TimeoutExpired(Exception):
         pass
 
@@ -911,8 +913,6 @@ def copy_example(self, name=None):
                 'example "{}" is not found as a file or directory'.format(example_path)
             )
 
-    Session = Session
-
     def getnode(self, config, arg):
         """Return the collection node of a file.