bpo-29762: More use "raise from None". (#569)

This hides unwanted implementation details from tracebacks.

Lib/chunk.py

@@ -64,7 +64,7 @@ class Chunk:
         try:
             self.chunksize = struct.unpack_from(strflag+'L', file.read(4))[0]
         except struct.error:
-            raise EOFError
+            raise EOFError from None
         if inclheader:
             self.chunksize = self.chunksize - 8 # subtract header
         self.size_read = 0