Move IO_REPARSE_TAG_* constants to _stat (#6760)

stdlib/_stat.pyi

@@ -1,3 +1,4 @@
+import sys
 from typing_extensions import Literal
 
 SF_APPEND: Literal[0x00040000]
@@ -76,3 +77,8 @@ def S_ISREG(mode: int) -> bool: ...
 def S_ISSOCK(mode: int) -> bool: ...
 def S_ISWHT(mode: int) -> bool: ...
 def filemode(mode: int) -> str: ...
+
+if sys.platform == "win32" and sys.version_info >= (3, 8):
+    IO_REPARSE_TAG_SYMLINK: int
+    IO_REPARSE_TAG_MOUNT_POINT: int
+    IO_REPARSE_TAG_APPEXECLINK: int