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So I have taken a look at the original patch that was provided and I have been able to update it so that it is compatible with the current release. I have also flipped the logic in the wrapping functions so that they take a `keepempty` flag (which is the opposite of the `prune` flag). 
I had to make a few extra changes since there are now some extra catches in things like PyUnicode_Split which spot that if len(self) > len(sep) then they can just return [self]. However that now needs an extra test since that shortcut can only be used if len(self) > 0. You can find the code here: https://github.com/markcbell/cpython/tree/split-keepempty
However in exploring this, I'm not sure that this patch interacts correctly with maxsplit. For example, 
 ' x y z'.split(maxsplit=1, keepempty=True)
results in
 ['', '', 'x', 'y z']
since the first two empty strings items are "free" and don't count towards the maxsplit. I think the length of the result returned must be <= maxsplit + 1, is this right?
I'm about to rework the logic to avoid this, but before I go too far could someone double check my test cases to make sure that I have the correct idea about how this is supposed to work please. Only the 8 lines marked "New case" show new behaviour, all the other come from how string.split works currently. Of course the same patterns should apply to bytestrings and bytearrays.
 ''.split() == []
 ''.split(' ') == ['']
 ''.split(' ', keepempty=False) == [] # New case
 ' '.split(' ') == ['', '', '']
 ' '.split(' ', maxsplit=1) == ['', ' ']
 ' '.split(' ', maxsplit=1, keepempty=False) == [' '] # New case
 ' a b c '.split() == ['a', 'b', 'c']
 ​' a b c '.split(maxsplit=0) == ['a b c ']
 ​' a b c '.split(maxsplit=1) == ['a', 'b c ']
 ' a b c '.split(' ') == ['', '', 'a', 'b', 'c', '', '']
 ​' a b c '.split(' ', maxsplit=0) == [' a b c ']
 ​' a b c '.split(' ', maxsplit=1) == ['', ' a b c ']
 ​' a b c '.split(' ', maxsplit=2) == ['', '', 'a b c ']
 ​' a b c '.split(' ', maxsplit=3) == ['', '', 'a', 'b c ']
 ​' a b c '.split(' ', maxsplit=4) == ['', '', 'a', 'b', 'c ']
 ​' a b c '.split(' ', maxsplit=5) == ['', '', 'a', 'b', 'c', ' ']
 ​' a b c '.split(' ', maxsplit=6) == ['', '', 'a', 'b', 'c', '', '']
 ​' a b c '.split(' ', keepempty=False) == ['a', 'b', 'c'] # New case
 ​' a b c '.split(' ', maxsplit=0, keepempty=False) == [' a b c '] # New case
 ​' a b c '.split(' ', maxsplit=1, keepempty=False) == ['a', 'b c '] # New case
 ​' a b c '.split(' ', maxsplit=2, keepempty=False) == ['a', 'b', 'c '] # New case
 ​' a b c '.split(' ', maxsplit=3, keepempty=False) == ['a', 'b', 'c', ' '] # New case
 ​' a b c '.split(' ', maxsplit=4, keepempty=False) == ['a', 'b', 'c'] # New case