Rebased elnuno's fix and retested: "Close CallbackFileWrapper.__buf once it's used."

[dwillie]

I noticed it hadn't been updated in a while so I just pulled it down and rebased it on the latest master, and re-tested it using the script @elnuno used (and pytest).

With @elnuno 's change (and a ~70mb file):

Using 36 MB on program end.
Mean memory use: 26 MB

Without:

Using 77 MB on program end.
Mean memory use: 189 MB

This is exactly the same changes as in:
#152

Just rebased as @ionrock had requested in 2018 with the hope of it getting merged.

EDIT: Obviously I don't deserve any credit for this, just trying to help get @elnuno 's fix in.