Make GridOut inherit from io.IOBase to be a fully "file-like" object (https://docs.python.org/3/glossary.html#term-file-like-object). Implement missing methods `readlines`, `writelines`,
`writable`, `fileno`, `flush`, `isatty`, `truncate`,
and property `closed`, following the spec
(https://docs.python.org/3/library/io.html#io.IOBase.writable).
Iterating over GridOut previously returned chunks, but IOBase specifies
that lines should be returned. Thus, the `GridOutIterator` returning chunks is removed
and GridOut simply uses the existing IOBase iterator implementation (returning `self`
in `__iter__` and using `readline` in `__next__`).
Additionally, iterating over GridOut previously did not move the "file pointer" along, i.e.
`next(iter(some_grid_out_object))` always gave the same result (the first chunk of the file)
as it would create a new iterator starting at the top of the file. This is now fixed as well, so
a first call to `next(iter(some_grid_out_object))` gives the first line, and subsequent calls return
the subsequent lines.
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