python: fix two obvious bugs in Transport class
python: fix two obvious bugs in Transport class

 - the `writev()` call needs an fd

 - the `_consider_closing()` method existed at one point during
   development, but never got merged.  Remove the leftover call to it.

Both of these issues slipped through because none of our tests managed
to trigger the situation where the queue backed up enough to trigger the
buffering code.  That will be added in a coming commit.

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