vpython: capture: use endswith instead of slicing

vpython/vpython.py

@@ -3142,7 +3142,7 @@ def capture(self, filename, capture_labels=True):
         if not isinstance(filename, str):
             raise TypeError("'filename' for Capture must be a string.")
 
-        if filename[-4:] != ".png":
+        if filename.endswith(".png"):
             filename += ".png"
 
         include_labels = "T" if capture_labels else "F"