Install diffeqpy in Python tests

[codecov[bot]]

Codecov Report

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

Project coverage is 26.27%. Comparing base (400f292) to head (3074473). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #639 +/- ## ========================================== + Coverage 26.19% 26.27% +0.07% ========================================== Files 55 55 Lines 4279 4267 -12 ========================================== Hits 1121 1121 + Misses 3158 3146 -12 ```

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