Minor bug in grain.python.RangeDataSource.__getitem__()

[albanie]

There may be a bug in grain.python.RangeDataSource.__getitem__().

Minimal code to reproduce:

import grain.python as pygrain 
x = pygrain.RangeDataSource(start=1,stop=10,step=2)
print(list(x))

Outputs:

---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
[<ipython-input-5-88dd2177f112>](https://localhost:8080/#) in <cell line: 3>()
      1 import grain.python as pygrain
      2 x = pygrain.RangeDataSource(start=1,stop=10,step=2)
----> 3 print(list(x))

[/usr/local/lib/python3.10/dist-packages/grain/_src/python/data_sources.py](https://localhost:8080/#) in __getitem__(self, record_key)
     92   def __getitem__(self, record_key: SupportsIndex) -> int:
     93     record_key = record_key.__index__()
---> 94     assert record_key >= 0 and record_key < self._len
     95     return self._start + record_key * self._step
     96