[3.9] bpo-43757: Make pathlib use os.path.realpath() to resolve symlinks in a path (GH-25264) (GH-135035)

Also adds a new "strict" argument to realpath() to avoid changing the default behaviour of pathlib while sharing the implementation.

(cherry-picked from commit baecfbd849)

Co-authored-by: Barney Gale <barney.gale@gmail.com>

Lib/ntpath.py

@@ -622,7 +622,7 @@ def _getfinalpathname_nonstrict(path):
                 tail = join(name, tail) if tail else name
         return tail
 
-    def realpath(path):
+    def realpath(path, *, strict=False):
         path = normpath(path)
         if isinstance(path, bytes):
             prefix = b'\\\\?\\'
@@ -647,6 +647,8 @@ def realpath(path):
             path = _getfinalpathname(path)
             initial_winerror = 0
         except OSError as ex:
+            if strict:
+                raise
             initial_winerror = ex.winerror
             path = _getfinalpathname_nonstrict(path)
         # The path returned by _getfinalpathname will always start with \\?\ -