GH-110109: pathlib ABCs: do not vary path syntax by host OS. (#113219)

Change the value of `pathlib._abc.PurePathBase.pathmod` from `os.path` to
`posixpath`.

User subclasses of `PurePathBase` and `PathBase` previously used the host
OS's path syntax, e.g. backslashes as separators on Windows. This is wrong
in most use cases, and likely to catch developers out unless they test on
both Windows and non-Windows machines.

In this patch we change the default to POSIX syntax, regardless of OS. This
is somewhat arguable (why not make all aspects of syntax abstract and
individually configurable?) but an improvement all the same.

This change has no effect on `PurePath`, `Path`, nor their subclasses. Only
private APIs are affected.

Lib/pathlib/_abc.py

@@ -1,7 +1,6 @@
 import functools
 import io
 import ntpath
-import os
 import posixpath
 import sys
 import warnings
@@ -204,7 +203,7 @@ class PurePathBase:
         # work from occurring when `resolve()` calls `stat()` or `readlink()`.
         '_resolving',
     )
-    pathmod = os.path
+    pathmod = posixpath
 
     def __init__(self, *paths):
         self._raw_paths = paths