MAINT: py3k: remove os.fspath and os.PathLike backports (#17473)

Since we no longer support Python 3.5 and below, this code is unreachable.

As with the rest of `py3k.py`, we leave behind the aliases to prevent breaking downstream code.

numpy/compat/py3k.py

@@ -133,54 +133,5 @@ def npy_load_module(name, fn, info=None):
 
 
 # Backport os.fs_path, os.PathLike, and PurePath.__fspath__
-if sys.version_info[:2] >= (3, 6):
-    os_fspath = os.fspath
-    os_PathLike = os.PathLike
-else:
-    def _PurePath__fspath__(self):
-        return str(self)
-
-    class os_PathLike(abc_ABC):
-        """Abstract base class for implementing the file system path protocol."""
-
-        @abc.abstractmethod
-        def __fspath__(self):
-            """Return the file system path representation of the object."""
-            raise NotImplementedError
-
-        @classmethod
-        def __subclasshook__(cls, subclass):
-            if PurePath is not None and issubclass(subclass, PurePath):
-                return True
-            return hasattr(subclass, '__fspath__')
-
-
-    def os_fspath(path):
-        """Return the path representation of a path-like object.
-        If str or bytes is passed in, it is returned unchanged. Otherwise the
-        os.PathLike interface is used to get the path representation. If the
-        path representation is not str or bytes, TypeError is raised. If the
-        provided path is not str, bytes, or os.PathLike, TypeError is raised.
-        """
-        if isinstance(path, (str, bytes)):
-            return path
-
-        # Work from the object's type to match method resolution of other magic
-        # methods.
-        path_type = type(path)
-        try:
-            path_repr = path_type.__fspath__(path)
-        except AttributeError:
-            if hasattr(path_type, '__fspath__'):
-                raise
-            elif PurePath is not None and issubclass(path_type, PurePath):
-                return _PurePath__fspath__(path)
-            else:
-                raise TypeError("expected str, bytes or os.PathLike object, "
-                                "not " + path_type.__name__)
-        if isinstance(path_repr, (str, bytes)):
-            return path_repr
-        else:
-            raise TypeError("expected {}.__fspath__() to return str or bytes, "
-                            "not {}".format(path_type.__name__,
-                                            type(path_repr).__name__))
+os_fspath = os.fspath
+os_PathLike = os.PathLike