Make retrieve_pdb_file() work on Python 3.

This applies the fix outlined by Michiel de Hoon in Redmine #3382.

Bio/PDB/PDBList.py

@@ -243,7 +243,7 @@ def retrieve_pdb_file(self,pdb_code, obsolete=0, compression=None,
         # Uncompress the file
         gz = gzip.open(filename, 'rb')
         out = open(final_file, 'wb')
-        out.writelines(gz.read())
+        out.writelines(gz)
         gz.close()
         out.close()
         os.remove(filename)