pythonGH-110109: pathlib ABCs: drop use of warnings._deprecated() (python#113419)

The `pathlib._abc` module will be made available as a PyPI backport
supporting Python 3.8+. The `warnings._deprecated()` function was only
added last year, and it's private from an external package perspective, so
here we switch to `warnings.warn()` instead.

Author: barneygale

Lib/pathlib/_abc.py

@@ -395,9 +395,8 @@ def relative_to(self, other, /, *_deprecated, walk_up=False):
         if _deprecated:
             msg = ("support for supplying more than one positional argument "
                    "to pathlib.PurePath.relative_to() is deprecated and "
-                   "scheduled for removal in Python {remove}")
-            warnings._deprecated("pathlib.PurePath.relative_to(*args)", msg,
-                                 remove=(3, 14))
+                   "scheduled for removal in Python 3.14")
+            warnings.warn(msg, DeprecationWarning, stacklevel=2)
             other = self.with_segments(other, *_deprecated)
         elif not isinstance(other, PurePathBase):
             other = self.with_segments(other)
@@ -419,9 +418,8 @@ def is_relative_to(self, other, /, *_deprecated):
         if _deprecated:
             msg = ("support for supplying more than one argument to "
                    "pathlib.PurePath.is_relative_to() is deprecated and "
-                   "scheduled for removal in Python {remove}")
-            warnings._deprecated("pathlib.PurePath.is_relative_to(*args)",
-                                 msg, remove=(3, 14))
+                   "scheduled for removal in Python 3.14")
+            warnings.warn(msg, DeprecationWarning, stacklevel=2)
             other = self.with_segments(other, *_deprecated)
         elif not isinstance(other, PurePathBase):
             other = self.with_segments(other)