[issue1751] Fast BytesIO implementation + misc changes

[Alexandre Vassalotti]

Alexandre Vassalotti <alexandre at peadrop.com> added the comment:

I think that check in write_bytes() is a guard to avoid resize_buffer()
from truncating the string stored in the buffer. However, I am not sure
if it is still necessary.

I don't know why help() doesn't work on BytesIO instances. But, the
problem doesn't seems to be in my code, since `help(sys.stdin)` doesn't
work either.

Finally regarding test_StringIO, see msg59460. Anyway, test_StringIO is
superseded by test_memoryio included my patch and should be removed from
the stdlib.

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