`_points_equal` incorrectly compares floats with `is not`

Nobody has claimed this yet.

I noticed add_ring is not treating my closed ring like a closed ring:

```python
ipdb> pt1
(19188372.37109346, -5402212.477493137)
ipdb> pt2
(19188372.37109346, -5402212.477493137)
ipdb> n
> /Users/cdestigter/checkout/sno/sno/pyvendor/vector_tile_base/engine.py(314)_points_equal()
313 if pt1[0] is not pt2[0] or pt1[1] is not pt2[1] or (self._has_elevation and pt1[2] is not pt2[2]):
--> 314 return False
315 return True

ipdb> pt1[0] == pt2[0]
True
ipdb> pt1[0] is pt2[0]
False
```

`is not` isn't the appropriate comparator here, it shoudl be `!=`. python doesn't guarantee two equal floats are the same float.

Open sno/pyvendor/vector_tile_base/engine.py and inspect _points_equal around line 313. Replace the identity comparison with value comparison, then run the project's existing tests and verify that equal float coordinates allow a closed ring to be recognized.

Written by the indexing model from the issue text.