fix stack deprecation warning

[jaydenpersonnat]

Small fix to resolve deprecation warning pandas.Dataframe.stack. Pandas has a new implementation of stack, which is set using future_stack=True. future_stack does not exist as a parameter in the pandas version compatible with 3.9, so Python version is checked when stack() is being used.

There's several warnings coming from numpy for python>3.9, but those seem to be internal issues with the current version of the library using deprecated pandas methods, so are probably benign.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 70.98%. Comparing base (89e5674) to head (08c620a).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## development #225 +/- ## =============================================== + Coverage 70.96% 70.98% +0.02% =============================================== Files 35 35 Lines 3220 3223 +3 =============================================== + Hits 2285 2288 +3 Misses 935 935 ```

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