Use the content encoding to decode the header

When decoding the header to put it back at the top of the contents of
the file, use the same encoding used in the content. This should be a
better "guess" that using the default value

Signed-off-by: Antonio Ossa Guerra <aaossa@uc.cl>

src/black/__init__.py

@@ -811,8 +811,8 @@ def format_file_in_place(
         raise ValueError(
             f"File '{src}' cannot be parsed as valid Jupyter notebook."
         ) from None
-    src_contents = header.decode() + src_contents
-    dst_contents = header.decode() + dst_contents
+    src_contents = header.decode(encoding) + src_contents
+    dst_contents = header.decode(encoding) + dst_contents
 
     if write_back == WriteBack.YES:
         with open(src, "w", encoding=encoding, newline=newline) as f: