Avoid positional dependence when dropping items

Also bump version.

Fixes #1 
It is still needed, since items has to be mutated in-place for pytest to honor it.  The `deselected` list is just used by pytest to say how many cases were deselected (at the end of a test run).

This could be a bit more efficient, though -- I realized after making the PR.

If we instead made a `kept` list, then this would result in items being mutated:

```
items[:] = kept
```

That'd cause the code to be O(n) instead of O(n**2).  But it probably doesn't matter, since it's a fast loop anyway.