Thanks!!  Code review:

Shouldn't closefd be passed as 1 in import.c?

I don't see the point of distinguishing between -1 and +1.  The block
"if (closefd < 0) { closefd = 1; }" looks rather silly.

In io.py, you should document that closefd must not be False when a
filename is given.

I think in _fileio.c, you can insist that the closefd argument is an int
(a bool will work anyway, as bool is a subclass of int).

I don't think we should warn when trying to close an unclosable fd; it
should really just be a no-op.  Also, if you are going to call
PyErr_WarnEx(), you should test its return value (it can raise an
exception!).

Please don't add trailing whitespace.