Added type hint

Author: radarhere

src/PIL/VtfImagePlugin.py

@@ -183,7 +183,7 @@ class VtfImageFile(ImageFile.ImageFile):
     format = "VTF"
     format_description = "Valve Texture Format"
 
-    def _open(self):
+    def _open(self) -> None:
         assert self.fp is not None
         if not _accept(self.fp.read(12)):
             msg = "not a VTF file"