means that select() on sockets > FD_SETSIZE (typically 1024) work again.
The patch makes sockets use poll() internally where available.

longobject.c: also fix an ssize_t problem
  <a> could have been NULL, so hoist the size calc to not use <a>.

_ssl.c: under fail: self is DECREF'd, but it would have been NULL.

_elementtree.c: delete self if there was an error.

_csv.c: I'm not sure if lineterminator could have been anything other than
a string.  However, other string method calls are checked, so check this
one too.

Proposed by Tim Peters.

is larger than FD_SETSIZE.

This can only be acheived with ulimit -n SOME_NUMBER_BIGGER_THAN_FD_SETSIZE
which is typically only available to root.  Since this wouldn't normally
be run in a test (ie, run as root), it doesn't seem too worthwhile to
add a normal test.  The bug report has one version of a test.  I've
written another.  Not sure what the best thing to do is.

Do the check before calling internal_select() because we can't set
an error in between Py_BEGIN_ALLOW_THREADS and Py_END_ALLOW_THREADS.
This seemed the clearest solution, ie handle before calling internal_select()
rather than inside.  Plus there is at least one place outside
of internal_select() that needed to be handled.

Will backport.

Probably should be backported.