Make formula expansion deterministic

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 98.91304% with 1 line in your changes missing coverage. Please review.

Project coverage is 87.45%. Comparing base (505703d) to head (ccc6908). Report is 6 commits behind head on master.

Files	Patch %	Lines
pyecsca/ec/formula/switch_sign.py	87.50%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #68 +/- ## ========================================== + Coverage 87.18% 87.45% +0.27% ========================================== Files 91 91 Lines 7745 7797 +52 Branches 1448 1454 +6 ========================================== + Hits 6752 6818 +66 + Misses 705 691 -14 Partials 288 288 ```

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