bpo-43012: remove pathlib._Accessor (GH-25701)

Per Pitrou:

> The original intent for the “accessor” thing was to have a variant that did all accesses under a filesystem tree in a race condition-free way using openat and friends. It turned out to be much too hairy to actually implement, so was entirely abandoned, but the accessor abstraction was left there.

https://discuss.python.org/t/make-pathlib-extensible/3428/2

Accessors are:

- Lacking any internal purpose - '_NormalAccessor' is the only implementation
- Lacking any firm conceptual difference to `Path` objects themselves (inc. subclasses)
- Non-public, i.e. underscore prefixed - '_Accessor' and '_NormalAccessor' 
- Unofficially used to implement customized `Path` objects, but once once [bpo-24132]() is addressed there will be a supported route for that.

This patch preserves all existing behaviour.

Lib/test/test_pathlib.py

@@ -1,3 +1,4 @@
+import contextlib
 import collections.abc
 import io
 import os
@@ -1459,7 +1460,7 @@ class _BasePathTest(object):
     def test_absolute_common(self):
         P = self.cls
 
-        with mock.patch("pathlib._normal_accessor.getcwd") as getcwd:
+        with mock.patch("os.getcwd") as getcwd:
             getcwd.return_value = BASE
 
             # Simple relative paths.
@@ -1738,21 +1739,18 @@ class _BasePathTest(object):
         # Patching is needed to avoid relying on the filesystem
         # to return the order of the files as the error will not
         # happen if the symlink is the last item.
+        real_scandir = os.scandir
+        def my_scandir(path):
+            with real_scandir(path) as scandir_it:
+                entries = list(scandir_it)
+            entries.sort(key=lambda entry: entry.name)
+            return contextlib.nullcontext(entries)
 
-        with mock.patch("os.scandir") as scandir:
-            scandir.return_value = sorted(os.scandir(base))
+        with mock.patch("os.scandir", my_scandir):
             self.assertEqual(len(set(base.glob("*"))), 3)
-
-        subdir.mkdir()
-
-        with mock.patch("os.scandir") as scandir:
-            scandir.return_value = sorted(os.scandir(base))
+            subdir.mkdir()
             self.assertEqual(len(set(base.glob("*"))), 4)
-
-        subdir.chmod(000)
-
-        with mock.patch("os.scandir") as scandir:
-            scandir.return_value = sorted(os.scandir(base))
+            subdir.chmod(000)
             self.assertEqual(len(set(base.glob("*"))), 4)
 
     def _check_resolve(self, p, expected, strict=True):
@@ -2199,6 +2197,7 @@ class _BasePathTest(object):
             p = self.cls(BASE, 'dirCPC%d' % pattern_num)
             self.assertFalse(p.exists())
 
+            real_mkdir = os.mkdir
             def my_mkdir(path, mode=0o777):
                 path = str(path)
                 # Emulate another process that would create the directory
@@ -2207,15 +2206,15 @@ class _BasePathTest(object):
                 # function is called at most 5 times (dirCPC/dir1/dir2,
                 # dirCPC/dir1, dirCPC, dirCPC/dir1, dirCPC/dir1/dir2).
                 if pattern.pop():
-                    os.mkdir(path, mode)  # From another process.
+                    real_mkdir(path, mode)  # From another process.
                     concurrently_created.add(path)
-                os.mkdir(path, mode)  # Our real call.
+                real_mkdir(path, mode)  # Our real call.
 
             pattern = [bool(pattern_num & (1 << n)) for n in range(5)]
             concurrently_created = set()
             p12 = p / 'dir1' / 'dir2'
             try:
-                with mock.patch("pathlib._normal_accessor.mkdir", my_mkdir):
+                with mock.patch("os.mkdir", my_mkdir):
                     p12.mkdir(parents=True, exist_ok=False)
             except FileExistsError:
                 self.assertIn(str(p12), concurrently_created)
@@ -2676,7 +2675,7 @@ class WindowsPathTest(_BasePathTest, unittest.TestCase):
         self.assertEqual(str(P(share + 'a\\b').absolute()), share + 'a\\b')
 
         # UNC relative paths.
-        with mock.patch("pathlib._normal_accessor.getcwd") as getcwd:
+        with mock.patch("os.getcwd") as getcwd:
             getcwd.return_value = share
 
             self.assertEqual(str(P().absolute()), share)